X+ = y

Question

generally what does this code mean [ x+=y]... a*=b.....t/=s .....,,, etc

Answer 1

x += y // x = x + y
x -= y // x = x - y
. . .

https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Compound_assignment_operators

Answer 2

it means make the mathematical operation using the two variables and store the result in the first variable.
so x += y means x = x + y
x *= y means x = x × y
x /= y means x = x / y .. and so on.