An investment fund is divided into a number of equal units which are then held for each investor. The value of a unit (the unit price) is calculated, usually daily, broadly representing the value of the whole fund divided by the total number of units. If the unit price is increasing, the value of the assets held in the fund is increasing.
Each time an investor pays a contribution into the fund, they’re allocated more units. Similarly, if they decide to withdraw or switch benefits from the fund, units are sold.