Subdivisions are dividing a beat into a number of notes, you can of course play two notes for each beat which would be 8th notes, dividing the beat by three would be triplets, four would be 16th notes etc. There is no limit but personally I only work up to dividing the beat into 6's - sevens are awkward and sound funny to me.

Many players (including me) tend to favour certain subdivisions and so learning to explore the less used ones is a very useful thing to practice. Fartlek is the Swedish word for 'speed play' and is more commonly associated with running training, but the term fits great for exploring the various subdivisions while improvising because with the same metronome tempo you will be exploring many speeds!

Subdivisions

Divide beat by	Name	Count
1	Whole notes	1
2	8th notes	1 - and
3	Triplets	1 - trip - let
4	16th notes	1 - e - and - a
5	Quintuplet	Hip - o - pot - o - mus
6	Sextuplet	1 - e - tip - e - let - e