It's often used to score things like movies or books where 10/10 is the best it could ever be, and 0/10 is the worst. 7/10 on this scale would mean a pretty good movie, not the best maybe, but well worth watching.

It's a ratio. Seven out of ten = 7/10 = 70% "7/10" is pronounced "seven out of ten" or "seven over ten". The former is used when talking about a ratio of a population/group (you could also use a percentage), and the latter is used when just talking about the number in mathematics. Some other examples: 2/4 (Two out of four) = 50% (fifty percent) 3/5 (Three out of five) = 60% (sixty percent)