What does the word inverse operation mean?

An inverse operation (for some operation) is, in a way, the opposite of another operation. For example, subtraction is the opposite of addition (if you add 7, then subtract 7, the subtraction will "undo" the addition - you get the original number back). Similarly, division is the inverse of multiplication, taking a root is the inverse of calculating a power, and the logarithm is also the inverse of calculating a power (the difference being that taking a root finds the unknown base, while taking the logarithm finds the unknown exponent).