If a pair of candidates occur in only two cells of a row, column, or region, these candidates form a hidden pair. Usually, it is difficult to find the hidden pair because of the other candidates in the grid cells where hidden pair is present. If you see the hidden pair, you can remove all other candidates from these two grid cells where hidden pair is located.
If three candidates occur in only three cells of a row, column, or region, these candidates form a hidden tripled. In this case, you can remove all other candidates from these three grid cells where hidden triplet is located.
If four candidates occur in only four cells of a row, column, or region, these candidates form a hidden quad. In this case, you can remove all other candidates from these four grid cells where hidden quad is located.