Preliminary tax bill
The bill for the first two quarters of the year, due February 1 and May 1. The new tax rate isn't set yet, so each of those payments is normally one-quarter of your total tax for the year before.
New Jersey example
Say your total tax last year was $8,000. Your February 1 and May 1 payments would normally be $2,000 each. Once this year's rate is set, the August 1 and November 1 payments cover the rest, so if this year's tax is $8,400, each is $2,200. By resolution, a town can instead make each early payment half of last year's second-half tax.
Official source
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.