A change the agency makes to a solicitation after it is posted: a new deadline, an answered question, a corrected attachment. Amendments are usually issued on an SF 30 and numbered in sequence, and most solicitations require you to sign and return every amendment with your proposal. Missing one, even a minor one, is one of the most common reasons a technically strong bid gets rejected before anyone reads it.

In a sentence

"Offerors shall acknowledge receipt of this amendment by signing and returning it with their proposal." Skip that signature and a strong proposal can be thrown out as nonresponsive before an evaluator ever reads past the checklist.