Liquid Steem are Steem that are tradable all the time. Let me explain the concept of "liquid" in crypto or other financial markets to make things more clear:
In economics, liquid assets refer to market assets that participant can quickly purchase or sell. Cash is the most liquid asset, because it can be exchanged for goods and services instantly with no loss of value.
There is no wait for a suitable buyer of the cash. There is no trade-off between speed and value. It can be used immediately to perform economic actions like buying, selling, or paying debt, meeting immediate wants and needs. Refer to https://en.wikipedia.org/wiki/Market_liquidity to learn more about liquidity.
Steem tokens that are always transferable are called liquid Steem. There is one other state of Steem which is Steem power. Steem power is not tradable. When liquid Steem is staked, it turns into Steem power.