The rules of Minesweeper are simple: to win the game you have to reveal all safe fields (squares) in the game. Safe fields are the squares that don't have a hidden mine underneath. Safe fields are either empty or contain a number.
A square showing a number indicates the number of mines in the directly adjacent fields (i.e. how many of the (up to) 8 surrounding fields contain mines). You can use the information of the number fields to conclude which closed squares hide mines.