One row at a time
Everything on this board is built from almost nothing: a row of cells, each either on (black) or off (white). Time flows downward — each row is computed from the one above it, so the picture you see is a history, with the first instant at the top.
To compute a cell, the automaton looks at just three cells in the row above: the one directly overhead and its two neighbours. A fixed rule says what the new cell becomes for each of the possible neighbourhoods. That's the entire machine.
The very first row is the starting condition. You can configure this starting condition in the Rule panel on the left — choosing between a single Center pixel, an inverted Negative center, or random Noise.
Start with the most famous opening move: a single black cell in the middle of an empty row, evolved with Rule 90. Watch it slowly, row by row.