There is really only one rule:

Fill in the grid so that every row, every column, and every 3 x 3 box contains the digits 1 through 9.
This means that - The digits to be entered are 1, 2, 3, 4, 5, 6, 7, 8, 9.

A row is 9 cells wide. A filled-in row must have one of each digit. That means that each digit appears only once in the row. There are 9 rows in the grid, and the same applies to each of them.

A column is 9 cells tall. A filled-in column must have one of each digit. That means that each digit appears only once in the column. There are 9 columns in the grid, and the same applies to each of them.

A box contains 9 cells in a 3*3 layout. A filled-in box must have one of each digit. That means that each digit appears only once in the box. There are 9 boxes in the grid, and the same applies to each of them.

You can't change the digits already provided in the grid. You have to work around them. Every puzzle has just one correct solution.