Memos let you make notes about any object in the simulation (so that the simulation is well documented).

When you hover the mouse over a simulation object on the simulation window, if you have entered a memo it will be displayed in a hover tip and in the status bar.

Memos are also available for some other (non-objects based) aspects of simulations - for example trials can be documented in a memo box in the trials dialog and File / simulation Details lets you write some text that will be automatically displayed whenever the simulation is opened.