notational question about :Goal

The manual defines
findall(+Template, :Goal, -Bag)
and the "Notation of predicate descriptions"
(http://www.swi-prolog.org/pldoc/man?section=preddesc), say ":"
indicates a meta-argument.

What's a meta-argument?  There is a reference to the discussion of
chapter 6 on modules, which includes 6.4 on meta-predicates, but that
seems to be something different.

I'm wondering about this because I was wondering if I could construct
a composite goal, perhaps like 
 (mount(A, B, W1), mount(A, B, W2), W1\= W2).
I realize that could be turned into a helper predicate.

I also notice the description of setof is
setof(+Template, +Goal, -Set)
for which Goal gets a "+" rather than a ":".  I know there are
differences in how setof and findall work, but I'm not sure what to
make of the distinction--probably because I don't know what ":" means!

Thanks.
Ross Boylan