It's a measure of how complex and active a language's word-building processes are.
This includes the different ways words can be modified, the number of meaningful parts (morphemes) within words, and how clearly those parts relate to their meaning.
Languages exist on a spectrum of morphological richness.
Others, like Finnish, pack a lot of grammatical information into single words.
Morphologically Rich Languages (MRLs) present unique challenges in natural language processing (NLP), often necessitating the development of specialized processing methods.