a screenshot would be nice.
but hold off doing doing so until you read my post.
we get this sort of question every day. the problem is that there are million of apps out now, each with their own icons. so, unless it is something popular like facebook or youtube, you will most likely get a guess from us. of course the easiest way to find out is to ask the person who the phone belongs to. but that is a discussion for another day.
so if you still want help from us, don't hold your breath on an actual answer. and if you still want help, can you post a screenshot of the icon in question?