This defines a left ideal in a noncommutative ring. Not much can be done with these objects at this point (as one can tell by the dearth of operations that take an NCLeftIdeal as input), , but eventually it will be a 'fully featured' object.
The object NCLeftIdeal is a type, with ancestor classes HashTable < Thing.
The source of this document is in NCAlgebra/NCAlgebraDoc.m2:1728:0.