White cells are dead, black cells are alive. Cells survive to the next generation if they have 2 or 3 neighbors in this generation. Dead cells are "Born" if they have exactly 3 neighbors. Any cell with fewer than 2 or more than 3 neighbors dies.
You can single-step the pattern by repeatedly clicking the "Step" button. Alternatively, you can auto-run by pressing the "Stepper" button. This will step through about 4 generations per second, except perhaps on a very slow computer. Click the "Stop" button to stop the auto-run.
This particular version (at present) has a bounded 41 x 41 universe. A toroidal universe is more commonly implemented. On page load there is a Light-Weight Spaceship at the upper left of the grid. You can use the "Clear All Cells" button and then enter patterns either by explicitly toggling the cells by left-clicking the mouse on them or by using the buttons on the right to load other patterns. Note that using the mouse to toggle cells doesn't work right when the Stepper is running, because you only have time to click the mouse once in the 1/4 second corresponding to a generation.
The "POOF" pattern can be used as a proof that in this game you can't go backwards in time. Think about it.
Another fine program by Grandpa Beeman.