In the financial markets liquid refers to liquidity; that is, how quickly and easily an asset can be converted to cash.

Real estate is much less liquid then, for example, bars of gold.

Gold can be converted to cash simply almost immediately at a precious metals dealer

Real estate, by contrast, requires a substantially longer period of time. Typcially a seller must retain the services of a broker, who in turn will advertise the property to locate a buyer. Unless wealthy, the buyer will approach a bank for a mortgage, which will require time for review, etc. The process of converting real estate to it's cash equivalent obviously takes much longer.

Any asset can be measured in terms of it's liquidity; this risk weighted asset, or RWA.