Context is a meta-data concept. Since todos/tasks can be related, for instance, to a specific project, it would be nice to express this semantic link : that's where Context rises up. You, as a user, will be able to link any kind of context name to a todo helping you to organize and give the semantic you want them to have.
Akonadi Backend : The context concept is stored into akonadi through the Akonadi Item Tag. An item holds a list of Tag that have a name and can be from diffents type. As a convention, a tag related to the type "context" would be zanshin related, its name, that will be displayed to the user, will depend on what the user as set.