Fill rate tells you, of the opportunities sent to the auction, how many came back with an ad. Low fill can point to thin demand in a geo, restrictive floors, or technical problems.
But fill is not a goal on its own. Lowering floors raises fill by letting cheaper demand clear; raising them does the opposite while lifting observed eCPM. Neither is automatically better. The number that tells you whether the auction is creating more value is revenue per eligible opportunity.
Also distinguish fill from show rate: an ad can be filled and still never be shown if the player leaves before the placement triggers.