An order may fill completely, fill only in part or remain unfilled depending on price and available counterparties. The fill record supplies the actual execution price and quantity used to calculate positions and realized results.
Sparse fills can make an automated strategy's returns irregular and difficult to scale. Evaluation should use executed trades rather than assume that every quoted opportunity was available at the desired size.
ELI5
An order fill means a submitted buy or sell request actually became a trade. Until that happens, the requested price is only an offer waiting in the market.
For example, a bot can post a one-cent bid many times but receive only a few fills because sellers rarely accept it. Those rare executions, not the number of orders posted, determine the real position and profit.
