it's a minus sign, it means subtraction
\(4x - (x+8) = \\ 4x - x - 8 = \\ 3x-8\).
there are many ways that you can think of this but the answer will always be the same.
10 - (3+5)
the answer is 10-8 = 2
which is the same as 10 - 3 - 5 = 2
4x - (8+x) = 4x - 8 -x = 3x-8
If there is a minus sign in front of a bracket you can get rid of the bracket and the minus sign by changing the sign of everything within th bracket. Minus signs become + and plus becomes -
4x - (8+x)
=4x - (+8+x)
Another way I like to look at it is that you are adding -1 times the bracket
4x - (x+8)
=4x+ -1*x + -1*8
=4x+ -x - 8
=3x - 8