A 57-cell is a puzzle that is made up of a bunch of little squares that are all connected together. The whole puzzle is a big square with 57 smaller squares inside of it.
Each of these smaller squares has a number inside of it. The numbers can only be between 1 and 9. And every row, column, and diagonals (the ones that go from the top left to the bottom right, and from the top right to the bottom left) must have all the numbers from 1 to 9 without repeating them.
It is like a big game of tic-tac-toe, but there are more boxes to fill in, and you have to make sure that each row, column, and diagonal has all the numbers from 1 to 9.
The puzzle was originally called "Number Place" but it became more popular as Sudoku. People all around the world love to solve these puzzles and challenge their brains!