The grid
The grid has 9 rows, 9 columns and nine 3×3 boxes. Some cells start filled in; these givens can't be changed. Every puzzle here has exactly one solution.
Sudoku is a logic puzzle on a 9×9 grid. Fill every empty cell with a digit from 1 to 9 so that each row, each column and each 3×3 box contains every digit exactly once. No maths is needed — the numbers are just symbols.
The grid has 9 rows, 9 columns and nine 3×3 boxes. Some cells start filled in; these givens can't be changed. Every puzzle here has exactly one solution.
Pick a digit and look at one box. Rows and columns that already contain that digit rule out cells in the box. If only one cell is left, that digit must go there.
A naked single is a cell where only one digit fits once you check its row, column and box. A hidden single is a digit that can only go in one cell of a row, column or box, even if that cell could hold other digits.
Turn on Notes to write small candidate digits in a cell. When you place a digit, it is removed automatically from the notes of cells in the same row, column and box.
A wrong digit shows in red and adds to the mistake counter. Undo steps back, Erase clears a cell, and Hint fills in one correct cell when you're stuck.