What message do guys (you barely know) want to convey when they touch a girl (arm) deliberately, yet seeming unintentional?

Flirting...

You just don't touch someone you don't want to touch.

If they are very gentle I would say it's a good sign.

What I don't understand about lads is though, they can give off all these signals that they "like" you, but there is a difference between "liking" someone for who they are and "wanting" someone for sex. How do you determine the difference between that one? And would lads wait around for the best part of a year for someone if they just wanted sex? Surely they urge for sex would get bigger and they would move on? Does anyone agree or can anyone help?