A task model is a more-or-less formal representation of a task, which in some cases may involve more than one person as well as digital and physical devices. One of the simplest and oldest is hierarchical task analysis, which decomposes a task into subtasks and sub-subtasks of those, etc. Other forms of task model may include representations of the domain, such as the objects (physical and digital) that the person interacts with and the triggers that cause an action or task to commence.
Graphical HTA of tea making. Source Chapter 15 Task Models Human–Computer Interaction, Dix, Finlay, Abowd, Beale (2004).