Instant trade is just what is says: there's no need to place an order and wait for a match. You can trade instantly and directly with Cryptal.
Just input the amount you want and click Buy or Sell. That's all there is to it! Liquidity is never an issue because you're buying directly from Cryptal.
Pick your crypto and pick your fiat. The price displayed reflects the current market with all fees included. Every 90 seconds the price is updated so you're never locked into a bad deal.