An asset is considered liquid when it can be converted to cash  without losing much (if any) of its value.  Therefore, cash is the most liquid asset possible.  If you've got cash, then you can buy goods or services as soon as you get to a store. 

A checking account is also a liquid asset, but it isn't as liquid as cash.  With a checking account you've got to cash a check or visit an ATM, or use a debit card.  Either way, there are a few extra steps involved in using your checking account to buy goods or services. 

Savings accounts, money market accounts, and other short-term investments are also considered liquid assets. 

Assets that are not close to being defined as liquid assets are often called fixed assets.  Your home is a good example of a fixed (non-liquid) asset.  It takes time to convert a home into cash.   Real Estate, cars, and other "property" are also fixed assets.