When angles are corresponding, they're essentially on the same side. It's sort of hard to explain, so I'll attach a drawing. The one on the left with the check mark is a corresponding angle, and the one on the right with the X is a same-side angle. Essentially, corresponding angles are on the same side and are also equal, unlike same-side angles, which are supplementary angles.
Let me know if you don't understand my explanation. -T.B.
