Vocabulary is the basis of a number of things - reading, pro-social behaviours, math, world knowledge and computational thinking just to name a few. It is a key readiness skill and is the foundation for the majority of the curriculum.
Vocabulary is an ‘emergent structure’ it depends on the experiences that you have and the language that you know. The visual representation she shared for this was the creation of an ant hill - something that evolves over time, no two people's vocabulary is the same.
A word in the lexicon means that the student knows - what it means, how it sounds, how it is spelled, how to use it grammatically, and whether affixes be added to it.