Hidden singles resolve more cells in a typical Sudoku puzzle than any other technique. After the obvious naked singles (cells with only one candidate) are gone, hidden singles keep the solve moving. Most medium puzzles are entirely solvable with these two tools alone.
A digit that has only one valid cell left in a particular unit — a row, column, or 3×3 box — even though that cell may still contain other candidates. The digit is "hidden" because you find it by looking at the unit, not at the cell.
In a row: Scan a row for a digit that appears as a candidate in only one cell. That cell gets the digit.
In a column: Same idea vertically. One candidate cell for a digit — place it.
In a box: Scan the 3×3 box. If a digit has only one cell remaining inside the box, it goes there regardless of what other candidates that cell holds.
A naked single is a cell with only one candidate — you see it by looking at the cell. A hidden single is found by looking at the unit and noticing a digit has nowhere else to go. The cell itself might show {2, 5, 9}, but if 9 is the only candidate for that digit in its row, 9 is placed.
Pick a digit, say 4. Find all 4s already placed on the board. Their rows and columns are eliminated. For each unsolved box, check whether 4 has only one remaining cell. If yes — place it. Then move to the next digit.
Working digit by digit through 1–9, then repeating, finds most hidden singles efficiently without tracking full candidate lists.
When hidden singles and naked singles stop working, the next step is usually naked pairs, then hidden pairs, then more advanced techniques like X-Wing.