The interest free period on a credit card is the maximum number of days offered between a purchase transaction and the full repayment date before the issuer will charge you any interest.
If you regularly pay off your credit card balance in full each month, a lengthy interest free period will give you more time to clear the balance in full without being charged any interest.
Look out for rewards as well as the longest interest free period - if you're a keen repayer then a rewards credit card could be earning you money for nothing.