I think this could use a fuller description.  For a new comer to RSpec, this sentence wouldn't mean much.  How about this:

RSpec has a few top-level constructs that allow you to begin describing behavior:

* `RSpec.describe`: define a named context for a group of examples
* `RSpec.shared_examples_for`: define a set of shared examples that can later be included in an example group
* `RSpec.shared_context`: define some common context (using `before`, let`, helper methods, etc) that can later be included in an example group

Historically, these constructs have been available directly off of the `main` object, so that you could use these at the start of a file without the `RSpec.` prefix.  They have also been available off of any class or module so that you can scope your examples within a particular constant namespace.

RSpec 3 now provides an option to disable this global monkey patching: `config.expose_globally = false`.  For backwards compatibility it defaults to `true`.