Definition of InLeft and InRight

Question

So in reading I have come across the terms "InLeft" and "InRight" and I am unable to find a concrete definition for it.

I have found it used in the specification for COQ, and in some notes on compilers (there it was refered to as INL and INR).

As far as I am able to ascertain it is to do with making a difference between two options. However the seems like a weird nomenclature for something so simple.