The principal is the money you borrowed, or whatever is left of it, as distinct from the interest charged for borrowing it. On a savings account it is the sum you deposited, the base on which the interest is calculated.
Each payment on a standard loan is split in two. Interest for the period comes first, worked out on the principal still owed, and whatever remains of the payment reduces the principal. Take £10,000 over five years at 6%, repaid at £193.33 a month. In the first month interest is £10,000 × 0.5% = £50.00, so £143.33 goes to principal and £9,856.67 is left. A month later the interest is £49.28 and the principal repaid £144.05. With every payment the split tips a little further towards principal, the pattern the CFPB describes for car loans, where the balance falls slowly at first and faster near the end. That gradual paying down is amortisation.
Two things follow. First, any money you pay above the instalment goes straight to principal, which lowers every later interest charge, so an overpayment in year one of a long mortgage saves more interest than the same sum paid in its last year. Second, the principal is not the same as the total you will repay: the gap between them is the interest, £1,599.68 on the loan above.
An interest-only loan is the exception. Its payments cover the interest alone and leave the principal untouched until the term ends, when it has to be repaid in one sum. The loan calculator shows how much of each payment reduces the principal across the whole term, and the mortgage calculator gives principal and interest as their own line, separate from property tax and insurance.