They are learning lots of new words in a short time.
Kindergarten is a time of tremendous language growth. Kids listen closely and use context clues to understanding new words. When that doesn’t work, most have no problem asking (and asking, and asking, and asking again).
What you should do: Talk a lot, and don’t worry too much about using too many big words. In fact, go out of your way to use new words in everyday conversation. Tell stories. Recite poetry. Play word games.