Composing functions means to use the output of the inner function as the input for the outer function.
In this case, the inner function is g(x)=8x, which means that Aurora sells 8 tickets per hour.
So, if we give 8x (number of tickets sold) as input to the function f(x)=0.5x, we'll get
Since the function f tells us how much Aurora gains per ticket, f(g(x)) basically means:
Aurora gains 0.5 dollars per ticket, and she sells 8 ticket per hour, so she gains 4 dollars per hour.