What is a "cash" investment?

"Cash" is an asset class that denotes short term interest bearing investments or interest bearing investments with a variable interest rate.

From the point of view of a small investor, cash could mean physical currency, but "cash" usually refers to a bank account, cash management trust or similar short term interest investment.

For a large investor, "cash" is a short term bond or "bill", usually one offered by a government but not necessarily. If a debt security of some sort has an expiry of less than one year then typically the amount of fluctuations in the capital value are small, even with relatively large interest rate changes. Thus in terms of risk these are the lowest risk investments, they are used for short term holding of funds while you are waiting to reallocate them to something else, like while you are trying to find a good share to buy, or while saving up for something.

Investment returns

As a "risk free" investment, the returns are the lowest of any asset class. Don't expect much more than a percentage point or two above inflation, and interest you earn is taxed fully as income. It has value for diversification reasons and liquidity reasons, but as a long term investment only barely beats stashing money into a mattress.