What is a Context in GTD?
A context defines what you need to be able to carry out an action. It can be either a tool or a place or a person needed to complete the action.
They are used to filter your Next Actions list, dividing it into smaller and more specific lists, which will help you focus on what you can really do at any given time.
Some of the most common contexts are: "Calls", "At Computer", "Errands", "At Office", "At Home", etc.