The game of Snooker is played with two players. These players take turns to play: a turn is over when the player fails to pot a ball on a shot; or fouls on a shot.
At the start of the game, the table is racked with 21 balls: 15 red balls, the one each of yellow, green, brown, blue, pink and black. In the game, a player scores one point for potting a red ball; two for a yellow, three for the green, and so on up to seven points for the black. In addition to these "object" balls, there is also the cue ball (white).
A turn consists of a number of shots: the turn is over after no balls are potted, or the player commits a foul shot. The legal shots are defined by the colour of the ball the player may go for. The cue ball must hit a ball of this colour; and it must do so before it hits any others. If any balls are potted, then they must be of this colour (multiple reds are permitted). If the player fails to hit the required ball; or hits a different ball first; or pots any other ball (including the white), then a foul is called: the opposing player is awarded points: a minimum of four; but if the player at the table was going for a higher valued ball (blue or above), then the number of points awared is equal to the value of that colour.
If a player successfully pots a ball of the required colour, then they are awarded the points associated with that colour. A turn must start with the lowest valued ball on the table (i.e. red, if there are any). After potting a red, the player is permitted to choose which ball to go for next, but that choice cannot be another red (this is known as "going for a colour"). After the colour it potted, it is returned to the table, and the player must again pot a red (if any remain): this red, again, gives the player the right (requirement) to go fo a colour. This sequence continues until no more reds remain on the table: the player then pots the colours in sequence -- colors are not replaced on the table in this phase of the game.
The game ends when there are no balls remaining on the table. The player with the most points wins. If you followed all that, then congratulations: you know how to play the game.