Scoring in Lucene is an advanced subject, but it is important to at least have a basic understanding of it. We will discuss the factors influencing Lucene's default scoring model and where to look for diagnostic scoring information. If this overview is insufficient for your interest, then you can get the full details at http://lucene.apache.org/core/4_8_1/core/org/apache/lucene/search/package-summary.html#scoring.

The important thing to understand about scores is not to attribute much meaning to a score by itself; it's almost meaningless. The relative value of an individual score to the max score is much more meaningful. A document scored as 0.25 might be a great match or not, there's no telling, while in another query a document scoring 0.80 may actually not be a great match. But if you compare a score to another from the very same search and find it to be twice as large, then it is fair to say that the query matched this document twice as well. The factors influencing the score...