An introductory period is a set length of time at the beginning of a financial product, typically on a loan such as a mortgage, where there is a special offer, for instance a lower interest rate fixed for that time period (fixed rate period).
After this special offer period, your interest rate would change, typically on a mortgage to the lenders standard variable rate.
You should plan to never go onto the lenders standard variable rate after the introductory period as this is typically high. Plan to remortgage when your fixed rate period ends.
Learn more about mortgages, and the best time to remortgage.