It covers the bid time and place, bid security, how to ask questions and substitution rules. It also states how long bids stay open and the grounds for rejecting a bid.
Example: the instructions state the last day for questions and the last day to request a substitution before the bid.
Read it at the start and put its deadlines on your calendar. The question deadline and the substitution deadline come well before bid day. Subcontractors are bound by many of these rules through the general contractor. Note how long the owner can hold bids, because your price must stay good that long.