It’s that sweet spot when you aren’t expected to start making payments on your loan. You can certainly start making payments (if you got it like that), but you won’t get hit with late fees or other penalties if you don’t. You’ve already received the loan or line of credit by the time the grace period has started.
For student loans, a grace period can help you settle into post-college life before you’re expected to tackle loan repayment. If you have a loan that accrues interest, this period can help you get ahead with interest-only payments.