"All the world ‘s a stage, and Morphemes and Semantic features are the players…"( W. Shakespeare)
(Ok, Shakespeare did not say exactly that about the world stage, but what can you expect from a (your) professor who majored in Tropical Agriculture).
With children having an apparently innate ability to tune in to the structure of the language being spoken, semantic features in combination with morphemes seem to set the stage for the development of grammar.
In every child's environment, there are actors (parents), actions and objects etc. are the semantic features or elements that are related to the baby's needs. They are ordered through the structure and/or morphology of the language. The child is tuned in, at least initially, to that structure.
In English at the two morpheme stage, the child uses any two of the semantic features. Hence, you might hear in Stage I of Roger Browns Developmental Chart, the following two word sentences (on the next slide, please):