I'm seeing: += a lot. What does it mean?

Answer 1

It means sum and assignment. Example:
a = 1;
a += 1; // result: a == 2

It is the same as:
a = a + 1;

Answer 2

x += 5 is shorthand of x = x + 5

Answer 3

Thank you so much for your quick response!

Answer 4

That's an assignment operator,
which assign sum of left and right operands to left one.
So x += y is equivalent to x = x + y.

These are similar operators:
-= *= /= %= etc...