Bid leveling is the work of adjusting competing bids onto a common baseline — adding back the scope one bidder excluded, clarifying assumptions, and accounting for alternates — so you can compare them fairly. The lowest number is rarely the lowest price once you level.
It's where scope gaps get caught. A steel bid that looks $80K cheaper might simply have left out connections or erection; leveling exposes that before you award and inherit the gap as a change order. A clean bid tab is the deliverable that makes an award decision defensible.