An addendum can change a spec section, replace a plan sheet, answer bidder questions or move the bid date. Most bid forms make you list every addendum you received.
Example: Addendum 2 adds a second scoreboard to the gym and deletes the wall pads at the stage. A bid based on the original sheets is now wrong in two places.
Read every addendum, even the ones that look like they belong to other trades. Mark the changed sheets and spec pages. Update your counts before you price. Acknowledge each addendum by number on the bid form.