A single is a cell whose value is already forced. There are two kinds, and the difference between them is the difference between a puzzle you can finish and one you stall on halfway.
A naked single is a cell with exactly one candidate left. Its row, column and box between them contain eight distinct digits, so the ninth is the only thing that fits. The answer is sitting in plain sight — hence "naked".
Naked singles are easy to spot once pencil marks are in, because you're just looking for a cell with one small digit written in it. Without pencil marks they're much harder, which is why a lot of players stall at exactly the point where crosshatching stops working: the deductions are still simple, but they're invisible unless the candidates are on the page.
They also chain. Filling one naked single removes a candidate from up to twenty neighbours, which frequently creates the next naked single directly. On a Medium puzzle, one lucky placement can unravel a third of the board.
A hidden single is a digit that has only one available home within a unit, even though the cell it lands in has other candidates of its own.
In the row above, one cell lists 3, 7 and 9. On its own that tells you nothing. But scan the whole row for the digit 7 and it appears in no other cell's candidate list. The row needs a 7 somewhere. There's exactly one place it can go. So that cell is a 7, and its 3 and 9 can be struck out.
This is the crucial move, and the one beginners consistently miss. A naked single announces itself — a cell with one candidate is visually obvious. A hidden single is camouflaged by the other candidates in its cell. You will not find it by looking at cells. You find it by looking at digits within a unit.
Naked single asks: what can go in this cell?
Hidden single asks: where can this digit go in this unit?
They're the same logic viewed from opposite ends, and a full pass over a stuck grid means asking both. Most stalls are a hidden single that nobody looked for.
Going unit by unit and digit by digit is 27 units × 9 digits — too slow to do exhaustively every time you stall. Some shortcuts:
Both techniques depend on candidates being accurate, and stale pencil marks cause more wrong answers than bad logic does. Whenever you place a digit, remove it from the candidates of every cell in that row, column and box before moving on. Skip that housekeeping once and you'll eventually "find" a single that isn't there, and the contradiction won't show up for another dozen moves.
The Notes mode on this site tracks marks per cell; the discipline of clearing them is still yours.
Once neither kind of single is available anywhere on the grid, you need techniques that eliminate candidates rather than place digits. Those eliminations then create fresh singles, and the cycle restarts. Start with naked and hidden pairs, then pointing pairs and box/line reduction.