A mutual fund is a collection of stocks or bonds that an organization or an Asset Management Company (AMC) buys on your behalf.
The AMC takes the money from different investors and pools that money together to invest in different sectors to get a higher return with the help of a market expert. The AMC then distributes the investment and keeps some amount of the portion collected from the investors in the form of a unit (called units), and the investor is called the unit holder of that mutual fund.
https://blog.finology.in/mutual-fund/basics-of-mutual-fund-investment