The player can capture a stone by playing into its last liberty. In the diagram a white stone A is captured when black plays to B. Liberties are empty intersection connected to a stone by line.
A string of stones is captured by playing into string's last liberty. In the diagram white stones (A) are captured when black plays into B. A stone string consists all the stones connected by lines. A string does not connect diagonally.
One of the players takes black "stones" and the other player takes white "stones". The board is a grid of 9 by 9 lines. The game starts with 4 stones are on the board as shown in the diagram. Black player starts the game. Each player plays a single stone of their own color. A player can play to any empty intersection. A player may not pass. Once the stone is played it is never moved to another intersection.
If none of the opponent’s stones is in atari, then the player should check if any of his own stones are in atari. If players stones are in atari, he should try to increase their liberties so that the opponent cannot capture then in his turn.
There are more tactical explanations in chapters "connecting stones" and "capturing stones". It is usually good idea to play many capture go games before reading tactical advices.
