Inc increases the value of the given variable by 1, unless a number number is entered.
Ex:
/set %test 24
%test now has a value of 24
inc %test
%test now has a value of 25
You can also set how much you want to increase:
/set %test 30
/inc %test 5
%test is now 35
I hope you kinda understand, maybe some one else can explain it better
[Edit]
Bah, collective beat me to it