Liquidity is about the ability to trade without moving price too much
A liquid market has enough resting buy and sell interest that orders can be executed with relatively little price impact. A thin market has fewer orders available near the current price, so a larger order may consume several levels of the order book.
That is why two assets with the same percentage move can feel very different to trade.
The bid-ask spread is an immediate trading cost
The bid is the highest price a buyer is currently offering; the ask is the lowest price a seller is willing to accept. The difference is the spread. In highly liquid markets the spread may be small. In thin markets it can widen quickly.
A wide spread means a market order can begin at a disadvantage before the market has moved at all.
Slippage grows when speed and size meet thin liquidity
Slippage occurs when the execution price differs from the price you expected. It can happen because price moved while your order was being processed or because there was not enough volume available at the top of the book.
Large orders relative to available liquidity are especially vulnerable. Splitting an order or using limit orders can reduce some price impact, but each approach comes with tradeoffs such as incomplete fills.
Liquidity can disappear when you need it most
Normal conditions can create a false sense of security. During a sudden selloff, major news event, exchange problem, or liquidation cascade, spreads can widen and order books can thin rapidly.
Risk models should therefore consider stressed execution, not only average execution.
Use liquidity as a filter before you trade
Before entering, check the typical spread, recent volume, order-book depth, and how your intended order size compares with visible liquidity. Small-cap assets may require smaller positions or may not fit your strategy at all.
A trade setup is only practical if you can enter and exit it under realistic conditions.