When a girl wants to pleasure you by licking, sucking your penis. Often times, teens dont know how to do this, so they practice a lot. They often times forget to be moving her hand up and down on the part of the penis her mouth cant reach, which will help the ejactulation process. Some girls enjoy, some dont, luckily, my girl doesnt mind it at all.
Advice to all guys: while getting head, make sure to stay very focused and into it, and think about how fucking awesome it is while its happening, and just how good it feels, it will make it even better, and during the orgasm, enjoy! It is the best, but, you might not feel the best right after, like, you will feel a little unhorny, but thats normal. Make sure to love your girl, because she definately cares enough to do this to you. My advice: go down on her too, if she wants it.