Well, you see, when a person first meets a person they find attractive, it's generally considered bad form to simply rip their clothes off and roll around like a pair of weasels, if for no other reason than that you might be in a public place.

So society has developed a number of ways for people to subtlely display their interest in another person. Hopefully said person will pick up on these signals, and even display some of their own.

That's flirting.