When referring to a liquid company / instrument or liquid market, it is often a way of describing how easily you can trade the instrument in question in the market without affecting the price. Also known as market liquidity.
As a rule, when there is high demand and activity around an instrument, there is high liquidity in the market for the instrument in question. In such cases, it will be easy to find buyers and sellers, and the trading volume is often very large in such cases.
With low market liquidity, you will have a significant effect on the price if you want to make trades with large volumes. This can often be very unfortunate, as low market liquidity often happens when global stock markets fall a lot, and large players may have to sell because you get "margin calls".