I'll just add a little to PianoBilly's excellent explanation.

In a major key, which is what the C scale is in the explanation, there happens to be three major chords, three minor chords, and one other chord(*).

It also happens that the three major chords are that 1, 4 and 5; I, IV and V.

The three minor chords are 2, 3 and 6 and in the Roman numerals notation, they're usually written lower case: ii, iii and vi. Usually, but not always. So the 'sixth' that's been mentioned elsewhere in this thread is a minor chord.

(*) The last chord is "diminished" and slightly different. Something for later if you don't already know that. It's normally written as vii followed by a 'degree' symbol.

Jazz relative beginner, starting at a much older age than was helpful.
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