Reading a chord
A chord is named after its root, the note it is built up from in thirds, plus its quality: major, minor (m), diminished (°), augmented (+), and for four-note chords the kind of seventh (7, maj7, m7, ø7, °7). The notes can come in any order and any octave; C E G, E G C and G C E are all C major.
What changes is the bass, the lowest note. With the root in the bass the chord is in root position. With the third in the bass it is in first inversion, written C/E or I⁶; with the fifth it is in second inversion, C/G or I⁶₄. Inversions sound lighter and let the bass line move by step.
The roman numeral tells you the chord's job in the key. I is home, IV and ii lead away, V and vii° pull back home. That is why the same G major chord feels like a question in C major and like a resting point in G major.
Questions
How does it name the chord?
It looks at the different notes you hold (not how many times each one is doubled) and checks every one of them as a possible root against triads, seventh chords, sus chords, sixths and ninths. The lowest note decides the inversion: with the third in the bass it is a first inversion, with the fifth a second inversion, with the seventh a third inversion. Anything else becomes a slash chord such as C/D.
Why does it say C♯ in one key and D♭ in another?
The same key on the piano can be spelled two ways, and the right one depends on the key you are in. In D major the third of an A chord is C♯; in A♭ major the root of a D♭ chord is D♭. Pick a key, or leave it on Auto and the namer chooses the key where the chord makes the most sense, and spells every note the way it would be written in that key.
What are the roman numerals?
They show where the chord sits in the key: I is the chord on the first note of the scale, V on the fifth, and so on. Upper case means major, lower case minor, ° diminished. G major is V in C major and IV in D major, and that position, not the name, is what tells you how it wants to move. The function panel explains what each one does.
Can I use my MIDI keyboard?
Yes, in Chrome, Edge, Firefox and Opera. Press Connect MIDI and choose your keyboard as the input, then just play. The last chord stays on screen when you let go, so you can read it. Safari has no Web MIDI yet; there you can click the keys or use the computer keyboard.
Some chords show no roman numeral. Why?
Roman numerals are given for the chords a key normally uses: its own triads and sevenths, plus common borrowed chords and secondary dominants. A chord from outside that vocabulary, or a sus or ninth chord, still gets its name and notes but no numeral.