"People say jargon is a bad thing, but it's really a shortcut vocabulary professionals use to understand one another." - Erin McKean
In a nutshell, vocabulary words are classified into three Tiers. Tier I words are words used in daily conversation - words most primary students know well. Tier II words are essential to reading comprehension and appear in all curricular subject areas. Tier III words are necessary to understanding important concepts within a particular field of study, such as biology, or physics.
For a more detailed description of the three "tiers" of vocabulary words, read this discussion "Including Tier 2 Vocabulary Instruction in Curricular Materials" at the Achieve the Core website.