What does this mean?

f(x) = |x| represents the modulus function or absolute value function. It gives us the distance between any point and the origin. Since distance can never be negative so we always consider its positive value only. You can simply multiply the given value with ‘-1’(if the given value is negative).

Step-by-step explanation:

I think the answer is 30.

The first digit can be one of three digits {3, 6, 9} corresponding to the last digit being {1, 2, 3}, and for each of those three digits, the middle digit can be one of ten digits {0 - 9}, making 3 x 10 = 30 such numbers.
It is assumed that a 3 digit number is a number in the range 100-999, excluding numbers starting with a leading zero, eg 090 is not considered a 3 digit number (though it would be a valid 3-digit number for a combination lock with 3 digits).