Tax grace period
The short window after a due date when you can pay without interest. A town may choose to offer one, but state law caps the normal grace period at 10 calendar days. Longer interest-free periods are allowed only in special cases.
New Jersey example
Say a quarterly payment is due and your town offers the full grace period. Pay within 10 calendar days after the due date and no interest is charged. Pay after that and the town can charge interest all the way back to the due date. After certain declared natural disasters, a town can let you pay without interest until the first day of the next month.
Official source
P.L.2020, Chapter 34 (amends R.S.54:4-67, interest and grace period on property taxes)
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.