A Hidden Pair is the mirror image of a naked pair. If, in a row, the numbers 2 and 6 can only go in the same two squares, then those squares must be 2 and 6, even if their pencil marks show other numbers too. You can erase every other candidate from those two squares.
Hidden pairs are harder to see because the pair is buried among other pencil marks. Look for numbers that appear only twice in a unit, and check whether two such numbers share the same two squares.