1 Answer

That means the fraction can't be "reduced" any more (I'm forgetting the term for this....dang).

Example:    2/4    both  the numerator (N) and denominator (D) are divisible by "2"  so 2/4 can be reduced by dividing both N and D by 2 and the fraction is now 1/2.   1 and 2 are non-zero whole numbers and the only factor they share is 1 (1 x 1, 2 x 1).  SImilarly 4/16 = 1/4;   6/18 = 1/3;  5/25 = 1/5.  You keep "reducing" until you have the situation described in your question. 

3/10,   5/8,    7/12,    4/13   are all expressed in their lowest terms.  They meet the criteria of your quetion.

9/30,   15/24,   84/144,   16/52  represent the same amounts, but are not expressed in their lowest terms.  The N and D of each fraction have whole non-zero numbers OTHER than one that are factors.  These fractions are not expressed in the lowest possible terms.