A modification changes a contract after award, adding scope, adjusting price, extending a deadline, exercising an option, and is different from a solicitation amendment, which only changes things before award. Some mods are bilateral, both sides negotiate and sign, typically for new work or a price change; others are unilateral, the contracting officer alone can issue them, but only for changes the contract already allows for, like exercising a pre-priced option year. Mods are numbered in sequence, so keeping track of every one that touches your contract is part of knowing what you actually agreed to.

In a sentence

"Modification P00003 extends the period of performance by 90 days at no additional cost." Read every modification as closely as the original contract, since each one can quietly change your price, scope, or deadline on its own.