Naked single: exactly one candidate remains
Suppose an empty cell cannot contain 1, 3 and 8 due to its row; 2, 5 and 7 due to its column; and 4 and 9 due to its box. The only remaining digit is 6, which can be placed immediately.
A naked single examines one cell and eliminates every forbidden candidate using the three intersecting units.
Hidden single: only one place for a digit
Imagine a row with several gaps. Digit 7 is missing, but existing sevens in crossing columns rule out all but one gap. That cell must contain 7, even if it still has other pencil marks.
For every missing digit in a unit, check its possible positions.