What the mean of += ?

Answer 1

x += 5 is same as x = x + 5

Adding 5 to the variable x.

Answer 2

More:
perhaps x+=5 equal/same to x=x+5
But In some old compiler
x+=5 and x=x+5 is completly different

Answer 3

+= ;plus equal to;
example:

x+=y means x=x+y

same,