Compound interest is interest calculated on your original amount plus everything that amount has already earned. Because the base itself keeps growing, the growth accelerates over time instead of staying flat.
The contrast is simple interest, which is always calculated on the original amount only. Over a year the two barely differ; over thirty years the gap becomes enormous. This is why time in the market matters more than the exact rate for most people — and why high-interest debt is so punishing, since the same mechanism works against you when you owe rather than own.