A context is a type or grouping of sealed content. For example, you might have contexts for secret sales matters, confidential sales matters, proprietary research matters, confidential partner communications, and so on.
The rights defined for each context can be very different so, for example, you might have rights to open, edit, and print confidential sales documents, but only the right to open secret sales documents, and no rights at all for research documents. Different users have different rights in different contexts.
If you are the owner or creator of sensitive documents, you can work with the rights administrator (or, in Oracle IRM terms, the "domain manager") to create one or more contexts to protect that information. For example, if you are developing a new product, you might create contexts to protect the requirements, specifications, designs, market projections, competitive information, legal information, patents, and so on. Your domain manager can help you decide whether you need new contexts, or whether existing contexts are suitable for your information.