Binary

Deep down a computer stores everything in microscopic switches that are either off or on — 0 or 1. Binary is simply counting with those two digits. Each switch in a row doubles in value: 1, 2, 4, 8, 16… so the row 101 means 4 + 0 + 1 = 5.

Everything digital rides on this trick: numbers, letters (each has an agreed number), pictures (pixels are numbers), sound, video. When someone says "it's all ones and zeros", this is exactly, literally what they mean.