This emoticon literally means "heart" since that is what it looks when viewed sideways. In IM language though, it's a less serious way of saying somebody you "love" them, or at least "care for" them. Even if you do really "love" somebody, you don't want to say it every minute and every day -- it gets old and may even scare him or her. So, use "<3" for normal every day conversations, reserving "love" during the times you really want the other to know you mean it.
(Sample IM chat)
you: ok, i'm going to sleep now.
her: nitey nite <3
you: <3 you too.