You use them the same you would use, say these:
local a = 0
a = a + 1
local b = 11
a = a - 1
You can shorten these to
a += 1
b -= 1
and a will be incremented by 1, b decremented by 1
Luau also introduces *=, /=, %=, ^=, ..= which are multiplication, division, modulus, exponentation, and concatenation respectively.