Block and lot
The pair of numbers that identifies a parcel on a town's tax map. Each block bounded by public roads gets a block number, and each parcel inside the block gets a lot number.
New Jersey example
Say your property is Block 12, Lot 7. That pair points to one parcel on your town's tax map and matches the parcel ID on the town's assessment list, so you can use it to look up your assessment. Every town must keep a tax map, except townships with fewer than 2,500 people, which may choose to.
Official source
Handbook for New Jersey Assessors (NJ Division of Taxation, revised July 2024)
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.