Basically, 'the principal' is the name that banks and lending institutions call the actual amount of money that a customer/borrower receives when they take out a loan. That's it.
For example, if you get a loan for $35,000, 'the principal' will be $35,000.
'The principal' does not include the interest or fees you will have to pay the bank or lending institution for getting a loan from them. For example, on a loan of $35,000, when you add interest and fees, you will probably repay the bank $40,000 for the loan.
To see how much money you will actually have to pay back for a loan, you should look at the loan's 'Annual Percentage Rate (APR)' or ask the bank.Preferential Interest Rate, Redemption Penalties, Annual Percentage Rate.
To learn more vocabulary connected to loans, you can do a free online exercise on bank loan vocabulary.