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What is the root note?
The note a or chord is built from and named after — the one that feels like 'home.' In A blues, A is the root; every other note is measured from it.
The root is the starting point. Play a scale from its root and it sounds settled; land on the root at the end of a phrase and the music feels finished. That is why scales and chords are named after theirs — 'A blues,' 'C major' — and why the root is always 1. Musicians also call the home note of a whole piece its tonic. Change the root and keep the same pattern of distances above it, and the scale moves to a new key without changing its character.