A loop is a set with some binary operation on it which satisfies some axioms. That binary operation is usually called "multiplication" but that's just a name. In particular, it's perfectly fine to instead call it "addition" if that makes sense in a certain context. So, that's all that's going on here: there is an operation called "addition" on the set $$R$$ and we're using it as the operation for a loop structure on $$R$$. We then refer to this loop as the "additive loop" of $$R$$ to distinguish it from other loop structures we might have relating to $$R$$.