It's not. A triplet means three notes in a space where there would "normally" be two.
Crash course in music theory:
Most current stuff is counted as 4/4, meaning four quarter notes to the measure. Each quarter note represents one beat in that case. Normal math applies, so a note twice as long as a quarter note is a half note and a note half as long as a quarter note is an 8th note. Like I said before, a triplet fits three notes in a space where there would normally be two. So if two eight notes make one quarter note, an eight note triplet does too. You just divide the quarter note into three equal parts. Similarly a quarter note triplet is as long as one half note (two musical beats in a 4/4 measure).
Hope that doesn't leave your brain spinning. These might help:
https://goo.gl/images/YqqARFThat one starts with a whole note, then half notes, quarter notes, eights and finally sixteenth (there are more, you could basically keep dividing as much as you want).
A triplet example is in this next image:
https://goo.gl/images/RydpJz