Explanation:
it means that the graph of the function above y=x must be the mirrored image of the graph under y=x.
it means that the function applied to itself gives x.
f(f(x)) = x
e.g.
f(x) = 5 - x
f(f(x)) = 5 - (5 - x) = 5 - 5 + x = x
so, this f(x) is its own inverse.