The arity of a functor is the number of arguments it takes. For example, the arity of likes/2, as in likes(jane, pizza), is 2, as it takes two arguments, jane and pizza.
ArityExampleCould Mean …
0linux.a bit like #define LINUX in* C
1happy(fido).Fido is happy.
2likes(jane, pizza). 
3gave(mary, john, book).Mary gave John a book.

* hard to use in practice in Prolog: if the code contains linux. then you could test for this, but if it doesn't, then testing linux in your code will trigger an "Undefined procedure: linux/0" error. You can work around this by declaring linux to be dynamic - i.e. putting the line

:- dynamic linux/0.
in your code (usually at the start).

Every fact and rule in a Prolog program, and every built-in predicate has an arity. Often this is referred to in descriptions of these facts and rules, by appending / and the arity to the name of the rule or fact. For example, the built-in predicate member/2 has arity 2.

A fact or rule can have more than one arity. For example, you might want to have two versions of make terms:

% make(X, Y, Z): X makes Y for Z
make(fred, chest_of_drawers, paul).
make(ken, folding_bicycle, graham).

% make(X, Y): X makes Y
make(steve, fortune).
make(kevin, mistake).
What we have here are make/3 and make/2.