Text modules are pre-defined answers that can fasten filling the Description field. When filling the Description field the user can simply select among th exisiting text modules and if necessary edit it in order to tune it to the specific answer. Currently these choices are applied at tenant levels (i.e. you cannot have different text modules per structure items/category or forms).
Go in the Configuration panel, then "Master Data" and "Text modules" and click on Create a new text module. You can decide whether it applies to events, tasks or instructions and then you will be able to write the desired text module.