Sometimes it's hard to explain to new players just what a SET is. Examples are quite helpful, of course. In addition, I sometimes find it easier to introduce an intermediate concept which I call a match.
A match is when three cards are either all the same or all different on a single property. For example, if all three cards have the same shapes on them, or all three cards have different shapes, then the three cards match on the shape property.
A SET is three cards that match on all four properties.