What does it mean when a guy touches the small of your back?

There's a guy who I think might be interested, but I'm not sure sometimes because I second-guess myself.

Before he has come up behind me and put his hand on the small of my back when asking me something. Initially I was kind of surprised. To me, that doesn't seem like something that guys "just do".

Would most guys just feel like they could just do that to any girl? I mean, I have to say, being that its an errogenous zone and all...I kind of liked it. =P