1 is home. Every other chord is counted up the scale from it, so the same numbers work in any key. Practice until a 4 or a 6m comes as fast as a letter.
On iPhone or iPad? Turn off your ringer (silent switch or Control Center mute) or you won't hear any sound.
You get one bar of count-in, then one chord per bar. Say each number out loud as it lights up. A voice calls out the next number just before each change, and Test voice checks your sound. With Change it up on, the voicing, bass and drums shift every round, and a fill leads into each new round. When it feels easy, turn the letters off and play along on your instrument.
These drills cover 1 through 6m. A 3 or a 7 usually shows up as 1/3 or 5/7, which you can see on the Key chart tab.
In modern music a 3 is usually played as 1/3 and a 7 as 5/7. That means the 1 or the 5 chord with the 3rd or the 7th in the bass. The chart lists every number for reference, but the practice skips the diminished 7°.
Numbers show the chord type for you: 1, 4, 5 are major, 2m, 3m, 6m are minor, and 7° is diminished. Real charts add details on top of the number, like 4sus, 5/7 or 1/3 for a chord over a different bass note.