NAND datasheets say that “Blocks zero and one are valid and will be valid for at least 1,000 program-erase cycles with ECC”. What does this mean?
Cypress guarantees that NAND blocks 0 and 1 do not contain factory bad block marks. Also, Cypress guarantees that these two blocks will not go bad for at least 1,000 program/erase cycles. This enables booting to happen reliably from the first two blocks so that 1st stage boot loaders do not need logic for bad block management.
Bad blocks in NAND flash
If certain operations fail on all or a part of a NAND block, the block is a Bad Block. More details are available in these articles: