Gah! That makes no sense (to me). I still get it when we're talking surfaces...the runway, the line looking flat but coming around to the other side.
But not when were not on the surface of anything. =P

I dunno...maybe I'm making it harder than it is.
I picture myself floating in space with two impossibly powerful laser lights. And they're wrapped together perfectly side-by-side as to (appear) perfectly parallel. They're 1 cm apart at the source.
Now, in a geometrically spherical universe, once they get out far enough the two lines will be 2 cm apart, and 3, and eventually 1km apart?
If that's right, the next obvious and naive question is...what's pulling them apart? And the answer would be: nothing 'cept the curvature of space. Yes?

if I'm with it so far, the next question is...until what? If the universe is infinite, OK. But if it's finite, what's at the edge? A literal edge of space and then non-universe outside that?
But what's this "back of your head" description? How's that possible?
OK, I think I can get that in a spherical universe...I can imagine the laser lines separating more and more until they've curved back back to the source, and I can imagine that without there even needing to be an edge of the universe (ouch!!)

But that doesn't make sense with a flat, finite universe. That implies an edge and an end, yes?
Have I gotten closer?