Root

The root is the note a chord or scale is named from and measured against. Every other note in that chord or scale is understood as an interval away from the root, so the root is the reference point the whole structure hangs on. A C major chord is "C-something" because C is its root; the E and G are heard as a third and a fifth above that C.

It is the first thing a chord or scale name tells you. Say "D minor 7th" and you immediately know D is the root and the rest is built on top of it; say "G♯ major scale" and G♯ is the root that the other six notes are spelled from. Most of the time the root is also the lowest note you play - the bass - but it does not have to be. When some other chord tone sits in the bass instead, the chord is an inversion, and the root is still the root even though it is no longer at the bottom.

Root is not tonic

The root belongs to a single chord or scale. The tonic is a wider idea: the home note of a whole key. They often coincide - in the key of C major the tonic is C and the I chord's root is also C - but they answer different questions. The root asks "what is this chord built on?"; the tonic asks "what does this whole piece revolve around?"