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/1.0/6.6.5.3_jcr_score_function.html.
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