A norm is part of the calculation of a score that's used to measure relevancy. When we search, a score is calculated for each matching result. This score will then be used to sort the end results. The score is what we refer to as a relevancy score.
Norms are calculated per indexed Field. This is a product of index time calculation (based on TFIDFSimilarity) and lengthNorm (a calculated factor that favors a shorter document). The higher value can help boost the relevancy of a document, which means that the document will rank higher in search results.
To further influence the search results relevancy, Lucene allows for two types of boosting: index time boost and query time boost. Index time boost is set per indexed field. It can be used to promote documents based on certain field values. Query time boost can be set per query clause so that all the documents matched by it are multiplied by the boost. It's useful if a certain filter takes precedence over everything else...