It usually refers to writing fractions. For example - if you had the fraction of 48/96, you could simplify it many times (24/48, 12/24, 6/12 & 3/6) eventually arriving at it's simplest form of 1/2 - meaning it cant be reduced any further.
Other examples could be:
25/100 & 5/20 = 1/4
33/99, 22/66 & 11/22 = 1/3
etc.
2/9 is in its simplest form.
21/100 is the simplest form.
3/7 . There is no simpler form.
2/5
13/118
3/10
3 over 21
3/5
The simplest form of 5/10 is 1/2
5 over 7
in simpelest form the fraction of 0.100 is 1/10
43 and 33/100