Liquidity
The spread, order book depth and the impact of large orders on the price.
Liquidity is the ability to buy or sell a significant amount quickly without a substantial change in price.
The spread is the difference between the best selling price (ask) and the best buying price (bid). In liquid markets it is narrow.
A large market order “eats through” several levels of the order book, so the average execution price is worse than the best price.