Your question is a bit vague, but here's the basic answer:
Confluence uses the Lucene search engine.
If you want to know more about how to search in Confluence this page and the child pages will be useful.
A good example of one of the problems with using Lucene: https://jira.atlassian.com/browse/CONF-3036
If you want to know more about the Lucene engine: http://lucene.apache.org/core/
Does that answer your question?
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