What is the root note of a chord?

The root note is the note a chord is named after. It's the foundation the other notes in the chord are built around.

A G chord is named after G because G is the note that every other note/chord resolves to. It feels complete. The root note of an Em chord is E. A Cadd9 chord has C as its root. Whatever letter comes first in the chord name is the root note of that chord.