The waist/hip ratio percentage reflects how much body fat somebody is carrying around the waist , respectively the hips.  It's a very simple calculation. Means dividing the waist circumference with the hip circumference. 
For example: 72 cm waist/103 cm hip (28 inch/40 inch) equals  0.7 WHR.
The higher the percentage, the person is considered being overweight, obese. 
Also, this simple calculation can be an indicator of what type of body the individual has.  Apple shaped or more pear shaped, concerning  where the fat deposit mainly is placed on the body.

 How to measure correctly? 
Standing position with feet gathered. The waist circumference should be measured where the belly button is, and the hip circumference on the widest part of the hip.

 It is believed, that people above 0.95 % (in the case of men) and above 0.85% (in the case of women) are more exposed to health problems, due to the accumulation of fat tissue on the upper body, around the organs.  May this presumption be true or not, but we might consider lowering it if we fall into that category. 
Nevertheless, we must mention, that this ratio is also an indicator of beauty because a narrow waist is always suggesting a more fit person. For example, women with a narrow waist and wider hips are considered more attractive, as in the case of man, again the narrow waist, but this time together with wide shoulders are preferred by the opposite sex... which actually  not necessarily has to be the opposite nowadays or even old days. 

 The most pleasing ratio for men, concerning women, according to studies proved to be the 0.7. This is kind of a magical number which seems to overcome cultural and time barriers since even studies from the 1920s till present days indicate the same results. Science got itself involved in the matter, studied Playboy bunnies and Miss America winners and all the time came up approximately with the same result, namely 0.7. 
The number it tricky, because if a bigger is divided with a bigger number, the outcome is still the same, although the look may be lot different. For example, Marilyn Monroe and Kate Moss... You wouldn't even imagine them as same body shape or type... nevertheless, science says they are similar...   But to stick to science till the end with this matter, I would just quote Einstein: everything is relative.