The function represents the number of accidents (f(x)) per 50 million miles driven as a function of the driver's age (x).
f(45) indicates that you have to find the value of f(x) when x=45, to do so replace the equation of the function with the value of x and solve for f(x)
For x=45 years f(x)=190
f(45)=190; This value indicates that 45-year-old drivers had 190 accidents per 50 million miles driven.