Valve combinations run down the side; tightening the embouchure climbs the partials across the top. Each cell is the note that pairing plays — tap it to hear it. The blank triangle is where a note is better reached on a lower partial.
Tap a cell to choose a note, enable the mic, then play it on your trumpet. The tuner listens for the concert pitch (what comes out of the bell) and tells you if you are on it, sharp or flat.
Open valves play the natural harmonic series: C, G, C, E, G as you tighten. Each valve combination lowers that series by a fixed amount — 2nd valve by a semitone, 1st by a tone, 1+2 by three semitones, and so on down to 1+2+3 (six semitones). Because each harmonic only needs filling down as far as the harmonic below it, the higher partials use fewer combinations — that is the empty triangle. The same note often has more than one fingering; this shows the standard one.