Buyout is where the general contractor takes the estimate and actually awards the work — soliciting bids, leveling them, and executing subcontracts and purchase orders for every trade. It's the bridge between winning the job and building it.
The classic trap is the apparent low bid that has a scope gap: the number looks great until you realize the connections, the restripe work, or the controls weren't included. Leveling bids so you're comparing the same scope — apples to apples — is the whole game. Do buyout well and procurement stays ahead of the schedule; do it carelessly and you'll fight change orders all year.