A triad is a chord with three notes: a root, a 3rd and a 5th. C major is C, E, G. Most big chords you strum are just a triad with some notes doubled, so the triad is the core of the sound.
On the G, B and high e strings a triad fits under three fingers and sounds clear and bright. That's why rhythm players use these shapes to cut through a band without clashing with the bass.
Key: C major. The first shape has C on the G string (fret 5), E on the B string (fret 5) and G on the e string (fret 3): root on the bottom, so this is called root position. Index on the high e at the 3rd fret, then ring and pinky on the 5th fret of the G and B strings (or flatten the ring finger across both).
Major and minor
Lower the 3rd by one fret and the chord turns minor: the B string drops from the 5th to the 4th fret and C major becomes C minor. One fret, a completely different mood.
The second exercise moves the same shape to F (fret 10) and G (fret 12). Same shape, different root, different chord.
The common mistake is hitting the D string as well. Aim your strum at the top three strings only, and rest the side of your fretting index lightly against the D string to mute it. Practice each chord picked first, then strummed.