Lucene Index Definition has few nodes(indexRules, aggregates, analyzers, tika) and its relevant properties that are responsible for indexing desired content (node types and properties in which the content is held in the repository) for full text search.
This post illustrates about
Full text search scenario
OOB Lucene Full text Index - /oak:index/lucene
Steps to create custom Lucene Full Text Index with sample use case and demo video.