Fill

What Does Fill Mean?
The action of completing or satisfying an order for a security or commodity. It is the basic act in transacting stocks, bonds or any other type of security.

Investopedia explains Fill
For example, if a trader places a buy order for a stock at $50 and a seller agrees to the price, the sale has been made and the order has been filled. The $50 price is the execution price, which also makes it the fill price - it is the price that allows the transaction to be completed.