Found this cool explanation:

Let’s start with two quick definitions. PUA means Pick Up Artist. An opener is a conversational piece-a question or a statement or something-which used by a pick up artist to strike up a conversation with a girl or group of people containing girls.
Obviously, to attract a woman you need to get talking to her, which is why it’s so important to get your openers right. If they fail, the whole conversation fails and, therefore, your chances of attracting the woman are nil.

So…your openers need to be VERY strong and delivered very efficiently.

There are three main kinds of openers. They are:

These are opening questions or statements that relate to the situation and environment you and the girl you’re going to be talking to are both in and experiencing. The advantage of situational openers is that they tie you and the girl together with a unifying subject matter. The disadvantage is that they can often be a little boring and without emotion. Your job, therefore, is to make sure your situational openers are emotionally engaging and interesting to the girl.

These are usually questions that involve asking the girl’s opinion on a specific matter or topic. They’re good because there’s plenty of room for interesting and really engaging conversation off the back end of the opinion opener.

direct openers
These are opening statements and questions that don’t hide behind any real “reason”. They’re simply direct methods of starting a conversation with a girl and often take the form of a one-sentence ‘gambit throw down’. Basically, you say something to the girl that makes her respond, then develop your conversation with her from there.

Here are two examples of powerful openers. The first one is called the Bridging Technique. It allows you to start a conversation in a really normal (and even somewhat uninteresting way) to get the girl on your side and the dialogue alive. THEN you use the existing dialogue to bridge into a much more interesting and high-energy mode, which is what hooks the girl or group. For example:
Let’s say you’re in a bar or at a party, you could start the dialogue by saying, “Hi. How are you?” It doesn’t matter what she says. She’ll probably say something fairly non-committal, like “fine” or “okay, thanks”. The split second she answers, say, “What do you think of the crowd? The people here?” As you ask this question, look around at the men and women. She’ll do the same. This shared moment helps build the connection between the two of you. You’re doing something together. She’ll probably answer by saying something like, “Yeah, they seem nice” or “They’re okay, yeah”. Now you’ve talked for a few moments and a dialogue has been established between the two of you, you can transition into the real meat of your opener. The basic rule is, step up the intrigue and interest with what you say next. In this example, you could say, “Yeah, I think they’re all right. Actually… (move in a little closer to her like you’re going to tell her a secret), there’s one guy I’m not sure about. There he is! (point out someone who looks like a bit of a character. Maybe he’s dressed in a weird way or something.) Will you protect me if he comes up to us and asks us to pull his finger or something?”

You can get a little wacky, just keep things fun and unpredictable. She’ll join in and you’ll have mutually felt sexual chemistry before you know it. Just remember that a bridge opener is one which starts with an everyday comment or question that no girl will find hard to answer, then moves into a more flirty, unusual mode.

That is a good example of a situational opener. Now let’s look at an example of an opinion opener. This one is especially good because it gets the women laughing AND quite possibly DISAGREEING with one another, which is a perfect way to inject some real energy into the group, which now includes you. This distracts the members of the group from thinking, “How come this guy is now talking in our group?” They’re having too much fun to think anything like that. If anything, you’re making their group’s dynamic BETTER. It’s called the Umbrella Opener and it goes like this.
Approach the group and almost walk past them. Just before you pass them, turn back a little like something has just flashed into your mind-this decreases the feeling that you have specifically approached THEM over any other group. Without facing them dead-on (turn your body slightly away), say: “Hey, do you think it’s okay for a guy to carry an umbrella?” They will either say:
- NO
They won’t saying anything else. Here’s what to reply with in each case:
“Really? Even if he does this when he carries it?” Now pretend to hold an umbrella above your head and stick out your pinkie finger, like people do when they drink tea in a goofy way. They will laugh. If they still say yes, then ask one of the girls if she’d mind her boyfriend carrying an umbrella like that.
- NO
“What if it was raining, wouldn’t you want him to protect you from the rain?”
“Would you mind if your boyfriend carried one with his pinkie finger out like this?”
“Because me and my friends have been talking about masculinity and femininity. One friend says she (social value is added here!) thinks it’s cool, my other friend says he thinks it’s gay looking.

From any of these four positions you can continue the conversation easily, because the group will be divided on what they think. You can work off this by created some controversy. The group will banter amongst themselves. Then you can bridge into a new topic. Introduce yourself, ask how they know each other, etc.