Triplets
A small 3 bracketed over a group of notes means: fit three of them into the time that two would normally take. The eighth-note triplet — three notes in one beat — is by far the most common.
Count it "1‑&‑a", or "1‑trip‑let", or "1‑la‑li" — three even syllables per beat. Whichever you use, the three notes are equal.
This should feel natural to you: it's the same three-in-the-space feel as compound time (6/8), and the same as tigun in tabla — the beat carrying three even strokes.
Two, then three, in the same beat
The quarter-note triplet
The bracket works on any value. A quarter-note triplet fits three quarter notes evenly into the space of two beats.
Duple and triple, back to back
On the D string
The usual mistake
Playing the triplet as "long–short–short" (a dotted feel) or as "two notes plus a grace" — a triplet is three even notes. And rushing straight out of it back into the duple notes. Slow the whole thing right down until the three are genuinely equal, then bring the tempo up.