What does it mean that two angles are supplementary?

Two angles are supplementary if their sum is 180 degrees.
Or if we have to determine the supplement of an angle of A degrees it is another angle
of 180 - A degrees.

For example, when a line is intersected
by another line we get two sets of supplementary angles. Supplementary angles can lie on
separate lines, just the fact that their measure adds up to 180 degrees is sufficient,
e.g. any two angles of a rectangle are supplementary but they do not necessarily have to
lie on the same line.

Two angles are
supplementary if they add up to give 180 degrees.