a triplet is 3 notes played in the time of two.
example:

if you play 1/8 notes you play 2 notes per beat evenly like


1 2 3 4 1
x x x x x x x x x

if you play 1/8 note triplets you play 3 notes per beat evenly like


1 2 3 4 1
x x x x x x x x x x x x x
\-3-/ \-3-/ \-3-/ \-3-/

you get it? same goes for quintuplets and all other kinds of uneven groupings. Check out the polyrythmics tutorial in my trick-section.