When we express a probability as a ratio, the numerator expresses the number of desired outcomes, that is, the outcomes we want to know about.
And the denominator express the total number of possible outcomes, that is, the number of outcomes that can happen, desired or not.
For example, If we have 3 red balls among 10 balls, the probability of picking a red ball is 3/10 (3 is the number of outcomes we want: red balls. 10 is the total amount of outcomes possible: 10 balls in total).