The name triad comes from the three elements that it contains: a Root note (that note which names the triad upon which we add the other intervals) a third (major or minor) and a fifth (perfect or diminished).
The open C major chord shape is played across 5 strings, but there are actually only three different notes contained within it, with the Root note and major 3rd being doubled one octave higher on strings 2(B) and 1(E).
If you play strings 5, 4 and 3 of the open C chord you are playing a triad. The Root note, C, is the lowest in pitch and so this is called a Root Position triad.
When the notes are played within the same octave (as close as they possibly can be to one another) it is said to be in a closed voicing.
If any of the notes of the triad are raised or lowered by one or more octaves it is an open voicing (or spread voicing). We will explore these later on.
Whether an open or closed voicing, if we are playing a C major triad and C is the lowest note being played, it will be a root position triad.