Pre-openers go by a lot of different names, but the concept is very simple. Give the girl a cue that you're about to open her. A wave, a wink, or usually a simple "hey" or "hi" are all you need.

Girls are not expecting to be approached during their daily routine, so if you launch in with your opener, whatever it is, "Do you know where the starbucks is?", "That's a cool watch", etc, she may not immediately realize you're talking to her. This is how you get blanked, or best case scenario she looks up and says "What?" and you have to repeat yourself - never a good thing. Best to use a pre-opener like "hey", wait until she looks up and makes eye contact, then use your opener.