"Como esta?" means "How are you". You can reply by saying "bien" (good), "estoy bien" (I'm fine), or "no muy bien" (not very well). If you want to find out how your friend is, you can reply "y tu?" (and you?).
It's a Spanish phrase used in a variety of situations which basically means "How are you?" Although it is not uncommon for it to be used on a constant basis, you might find that it is somehow the equivalent of "What's up?" or other similar syntagms most of us use unconsciously.
