Subtraction is basically taking some "amount" away from a number and counting whats "left-over".
For example, If I have 10 dogs, and I give away 3 dogs, then now I only have 7 dogs left. This word phrase
could be rewriten in mathematical terms in such a way:
10 - 3 = 7.
Given some number x and y and z. "-" signifies that we want to "take-away" some number y from x.
x - y = z