The purpose of the game is to force the opponent make bad moves, such as giving you a corner. Of course, the opponent does not make bad moves, if good moves are available to him/her. So, the cornerstone of the strategy is to minimize the number of moves available for the opponent. Roughly speaking, the more stones you have on the board, the more available moves the opponent has. Hence, in the beginning and in the middle game, it is a good idea to have only a few stones on the board. In particular, your stones that are next to empty spaces give opponent moves, and it is ok to have stones on the board that are surrounded by other stones on all the eight directions.

In the end game, you of course want to maximize the number of your stones, and the end game is about reading sequences of moves.

A stable stone is a stone that cannot be flipped back by the opponent. Stones in the corners are stable stones, and often stones next to stable stones (such as the stones in the corners) are also stable. You should aim at getting stable stones for yourself.