Mocha sets the value of the this variable for the functions passed to describe and test/it.
This allows to dynamically configure the behaviour of the tests (skip tests, define timeout, ...)
Their website mentions the following methods:
this.skip
this.retries
this.slow
this.timeout
I wasn't able to find a "structured" API documentation. There are more methods available when checking the source code, but supporting only the documented methods would be a good start.