The principal is the original amount borrowed or invested, as distinct from the interest earned or charged on it.
On a loan, every payment splits between the two, and only the part hitting principal reduces what you owe. Early payments are mostly interest, which is why a balance seems to barely move at the start of a long loan. Any extra amount you pay usually goes entirely to principal — and because all future interest is calculated on that smaller balance, an early overpayment removes far more total cost than a late one.