A fill is the execution of an order: the point at which a buyer and seller are matched and the trade completes at the fill price. It confirms the price, quantity, and time of the trade.
A fill can be complete or partial. A complete fill executes the whole order quantity, while a partial fill executes only part of it, because there was not enough liquidity at the required price or size.
A fill is the result of execution, and execution is the process that produces it. The fill price is the actual price you trade at, and the gap between it and the price you expected is slippage.
You place a buy order for 1,000 shares at USD 20.00.
Only 600 shares are available at that price, so you receive a partial fill of 600 shares.
The remaining 400 shares stay active or are cancelled, depending on your order settings and the broker's rules.