Interest is the cost the borrower pays to the lender for the benefit of using the principal. Interest rates fluctuate daily and move according to the global economy, the US economy and the stock and bond market. Interest rates are expressed as a percentage. The Interest Rates are currently at all time lows with 30 year fixed at 3.5% interest rate. The Fed has said they will not raise interest rates until unemployment is less than 6% nationally, however there is no guarantee what they will do in the future. Th graph below was compiled from Historic 30 Year Fixed Rate Mortgage Data provided by FreddieMac.
See Rate Lock, Amortization