The context is an option that can be applied to tasks in Informant. It serves multiple purposes. Its an identifier, it is also another way for tasks to be grouped.
Below is a simple example of how to use a context with your tasks.
Lets say that you buy groceries at a place called The Food Place, at a specific address.
In Informant you have set several tasks among many projects to buy various items at that store.
If you have added the context The Food Place to all of your tasks that involve The Food Place you can go into Informant, into the task view and tap the context grouping then select The Food Place context and see all the tasks, regardless of due date, project etc. that involve The Food Place and you can complete the tasks that you wish to at that time for that place.
The iOS Informant will allow you to assign a specific location to the context that will fire an alarm when you are near the location alerting you to the fact that you have tasks that can be completed at the location.
Contexts will sync only via the Informant Sync service and Toodledo