When every cell is filled and no number repeats in any row, column or 3×3 box. Repeats are highlighted as conflicts while you play.
A puzzle is solved when two things are true at once: no cell is empty, and no number appears twice in the same row, column or 3×3 box.
What the game checks
As you play, a number that repeats in its row, column or box is highlighted as a conflict. The game does not tell you whether a number is the right answer, only whether it clashes with another number. A clash-free number can still be wrong until the grid fills up.
Why a clean grid is the answer
Every puzzle has exactly one solution, so a full grid with no repeats must be it.
If you can't finish
Check the cells near any conflict first, then try pencil notes. The solving techniques guide has the next steps, and how to play Sudoku has the full rules.