Refinancing a mortgage or "refi" for short means paying off an existing loan and replacing it with a new one. Borrowers often choose to refinance when the interest-rate environment changes substantially, causing potential savings on debt payments from a new agreement.
Other reasons why borrowers refinance:
To shorten/lengthen the term of their mortgage resulting in a higher/lower monthly instalment respectively.
To convert from a floating rate to a fixed-rate mortgage, or vice versa