The result returns a score for each row in the result set. The score contains a value that indicates a rating of how well the result node matches the query. A high value means a better matching than a low value. This score can be used for ordering the result.

eXo JCR Scoring is a mapping of Lucene scoring. For a more in-depth understanding, please study Lucene documentation.

jcr:score counted in next way - (lucene score)*1000f.

Score may be increased for specified nodes, see Index Boost Value

Also, see an example Order by Score