A quick, no-memorization tour of the words the course leans on: pitch class, interval, scale, chord, arpeggio, tonal center, and root. The one distinction worth catching now is root vs. tonal center — a root is local (the note a given chord or scale is named from), while a tonal center is global (the "home" note a whole passage pulls toward). Nothing to drill here — just enough shared vocabulary so the lessons ahead land cleanly.