Supporting characters may make a difference in the plot, but readers aren’t emotionally involved with them. They may cause a twist in the story, but they will not play a major role in shaping the story.
A supporting player does one or two things in the book and disappears. You may name supporting players and perhaps divulge a little bit of background, but they are not as important as major characters are.
Examples of a supporting character would be a man who discovers the body in a mystery, the crazy ex-girlfriend in a romance, or the guard of the Seat of Death in a science fiction book.