What is the meaning of zero in "3 miles below sea level" (we are doing integers)

Think of a number line. You have 0 in the center, then to the right of 0 you have positive numbers, and to the left of 0 you have negative numbers. In this case, think of sea level as 0. Anything above 0 (sea level) will be a positive number, and anything below sea level will be a negative number. So, 3 miles below sea level would be -3.