Does it just mean one team gives an asset(s) to another team for free, or is there more to it?

It usually means «absolutely nothing».

Future considerations is usually another way of saying nothing but it is occasionally not nothing

Per the CBA you aren't allowed to trade an asset for "nothing". These future considerations have to be something, but they can be as simple as a High Five, or a Dollar.

Mmm, I see, how about a $20 signing bonus?

When is it not nothing?

It'a never nothing. We just don't hear about it because it's something very simple and not news worthy