The correct interpretation of the function \( y = 15x + 12 \) is:
y is the cost and x is the number of months someone is a member of the gym; $12 is the enrollment fee and $15 is the monthly fee.
In this function, \( y \) represents the total cost, which includes the one-time enrollment fee of $12 and a monthly fee of $15 for each month of membership represented by \( x \).