"Triads" refer to chords that are comprised of 3 separate notes; the root note, a third and a fifth (with respect to the root note). The exact interval of the third and the fifth depends on the type of triad in question. There are 4 types of triads which are the major, minor, diminished and augmented chords.
The major triad consists of a major third, and a perfect fifth interval. This is symbolized in terms of semitones as 0-4-7. As an example, let us build a C major triad:
Here, the C is the root note. Then we take notes that are 4 and 7 semitones above the root note to form the C major triad.
The minor triad on the other hand is made up of a minor third and a perfect fifth, which is represented as 0-3-7 in terms of semitones. The C minor triad is shown below to clarify the concept of minor triads:
The diminished triad is comprised of a minor third and a diminished fifth, and is represented as 0-3-6. The C diminished triad is shown below:
The augmented triad contains a major third and an augmented fifth, which is 0-4-8 in terms of semitones. A C augmented triad is shown below: