To pay the principal means to repay the original amount of money borrowed or owed, without including any additional fees or interest charges. Full definition
The slices could be set up so that, say, $ 900 million worth are relatively safe investments, based upon homeowners paying the principal on their mortgages. (npr.org)
The amount goes towards paying the principal balance and interest, and is determined according to the down payment, term, interest rate and cost of the property. (fha.com)
Then instead of paying the principal down each month, you would instead invest in stocks. (actuaryonfire.com)