A scale is a family of notes that sound good together. Take C major: C D E F G A B. Stay inside those seven notes and almost anything you play will sound "right". That is why finding the scale is the whole point of this site · once you know the family, you can improvise without fear.
The key is the home of a song. When we say a song is "in G major", it means the song's melody and chords are built from the G major scale, and the note G feels like home · the place where phrases want to land and rest.
The root is the note a scale or a chord is built on · its ground floor. In C major the root is C; in a Dm chord the root is D. Starting and ending your phrases on the root makes your solo sound resolved instead of lost.
Take the same seven notes of a major scale, but start from a different one of them · you get a mode. Same ingredients, completely different mood. Start C major from D and you get D Dorian (jazzy minor); start it from A and you get A minor (sad). That is why this site shows seven modes for every key you find: seven moods for free, no new notes to learn.
A chord is several notes played at the same time. The most common kind is the triad: pick a note, skip one, pick, skip one, pick · like C, E, G. Three notes, one sound.
They describe a chord's mood, and the mood comes from the distances between its notes. Major chords sound happy and open (C), minor chords sound sad or thoughtful (Cm), diminished chords sound tense and unstable (C°) · they want to move somewhere else immediately.
A 7th chord is a triad with one more note stacked on top · it adds color: maj7 sounds dreamy, m7 sounds smooth and jazzy, 7 (dominant) sounds bluesy. A sus chord (suspended) replaces the middle note of a triad with its neighbor · sus2 or sus4 · so the chord floats, neither happy nor sad, waiting to resolve.
The chords you can build using only the notes of one scale. Every scale gives you exactly seven of them, one on each note. They are guaranteed to fit together · that is why the site lists them for every key: it is your safe palette for writing or jamming.
A chord progression is chords played one after another · the skeleton of a song. Verse, chorus, everything sits on top of a progression. We write them with roman numerals (I–IV–V) so they work in any key.
They number the scale's seven degrees, so a progression can be described without naming a key. In C major: I = C, ii = Dm, V = G. Uppercase means major, lowercase means minor, the ° sign means diminished. "I–V–vi–IV" in C is C → G → Am → F; the same recipe in G is G → D → Em → C. Same feeling, different key · that is the magic of numerals.
Because a handful of progressions carried most of popular music. I–V–vi–IV alone powers hundreds of hits; ii–V–I is the heart of jazz; the 12-bar blues is I–IV–V rearranged. Learn to hear these few shapes and half of the radio becomes sight-readable.
The same note, higher or lower. A piano has many C's · each one doubles the frequency of the one below. That is why the piano here offers 1 to 3 octaves: same note names, different heights.
The distance between two notes, measured in steps. The smallest one on a piano is a half step (E to F); two half steps make a whole step (C to D). Stack intervals and you get chords; walk them and you get melodies.