Diatonic triads are triads or chords that belong to a key. In this case, the key of C. The definition of a triad is 3 notes and the definition of a chord is 3 or more notes so you can call these either at this point. Once we add more notes to our triad we will want to call them chords.

We end up with a series of major and minor chords with the seventh one called diminished. These are the chords belonging to the key of C.

Because all major scales are built the same way :

W W H W W W H

the I, IV and V chords will always be major

the ii, iii, vi will be minor

and the vii will be diminished

Note we will use upper case roman numerals to indicate major and lower case for minor.