What is JCR:Score
What is JCR:Score?
If you are running a JCR search for page where the title contains "xxx", then the jcr:score is calculated when you use "contains" function in your query and the jcr:score gives the proximity of those pages matches your condition (that is your specific implementation).
The score is calculated using a pretty complex mathematical formula. You can read more about it at https://docs.adobe.com/content/docs/en/spec/jcr/1.0/188.8.131.52_jcr_score_function.html.