In math it usually means that when a value is put into a function, there is only one possible output. One value in produces one value out.
For example, think of the function y(x) = sin(x). Whatever value of x we choose there is just one corresponding value of y. If x = 45°, say, then y(45°) = 1/√2. One value in, one value out.
However, if we had the function y(x) = sin-1(x), then if x is 1/√2, there is more than one possible value for y - it could be 45° or 135° for example. One value in, more than one out.