The easiest way to understand what are triplet notes in music is to simply count out loud – one, two, three, one, two, three. When counting the first note (one) is accented. This is triplets.
Triplets as well as other notes have a duration. Let’s look at them in more details.
Triplets Duration
Whole Triplet
Let’s start with the whole triplet note. It is extremely rare to see on the music staff. If we take the time signature 4/4, the whole bar will take the whole triplet with a dot (the dot increases the note duration by 1,5 times).

Half Triplets
The 4/4 bar contains 3 triplets and looks like this on the musical staff:

Quarter Triplets

Here, for every 2 beats of the metronome with a time signature of 4/4, there are three notes.
Eighth Triplets

When we playing of the eighth triplets, each beat of the metronome contains for three notes. It is with the example of the eighth note that it is easiest to understand what triplets are.
Sixteenth Triplets
One metronome beat contains six notes.
The sixteenth triplets at a tempo of 50:

If we are talking about the duration of a triplet notes, then three triplets of the same duration are equal in sounding time to two ordinary notes of the same duration as the triplets. Below it is shown on the example of the eighth notes:

Here you can see that one beat includes 3 eighths triplets or 2 eighth notes.