In a Sudoku puzzle, if two digits are candidates for the same two cells in the same row and not for any other cells of that row, then such two digits must be in either one of these two cells and other candidates in those two cells can be eliminated. The same reasoning can be used for any columns and any 3x3 boxes. The technique is called hidden pair.
This technique can be applied to more than two digits. It is called hidden triplet if three digits are involved and hidden quad if four. The number of cells must be the same as the number of digits. But each of the digits involved need not be a candidate of all cells.
Consider the following partial Sudoku puzzles:
The sixth and seventh cells of this row have digits 2 and 3 as candidates besides others. Also these two digits are not candidates for any other cells in the same row. So these two digits 2 and 3 will occupy the sixth and seventh cells and all other candidates for these two cells can be safely eliminated.
The digits 3, 7 and 8 appear only in the third, fifth and eighth cells of this row. All other candidates in these three cells can be eliminated.