Press the spacebar to start the simulation. If you want a specific pattern, click on the cells before you've pressed space. Press 6 to clear the grid. A fun thing to do is to just draw a random pattern and just see what happens.
The simulation is John Horton Conway's game of Life. The way it works is there are two states a cell can have. Alive and dead. The way it works has to do with neighbors. Each cell has 8 neighbors. If 1 or less of the neighbors are alive, the cell dies from loneliness. If there are 4 or more alive neighbors, the cell dies from overcrowding. If there are 2 or 3 alive neighbors, the cell stays alive. If a dead cell has 3 alive neighbors, the cell becomes alive again.