The ask is the lowest price sellers are currently offering. The bid is the highest price buyers are currently offering.
Example: the market shows a bid of $2,490 and an ask of $2,500. Buyers are offering $2,490, while sellers are asking $2,500.
For a buy limit order to fill, your limit price must be at or above the current ask - a buy limit at $2,500 can fill because the ask is $2,500, but if the ask rises to $2,510, it cannot fill until the ask comes back to $2,500 or lower.
For a sell limit order to fill, your limit price must be at or below the current bid - a sell limit at $2,490 can fill because the bid is $2,490, but if the bid drops to $2,480, it cannot fill until the bid rises back to $2,490 or higher.
The amount that fills at any given moment depends on how much size is available at your limit price or better, not just whether the price has technically been reached. See: Why was only part of my order filled?