Relevance in SEO is the semantic correspondence of the search query to the search response of the document.
The relevance of documents to queries on the Internet is automatically determined by the algorithms of search engines. In the simplest of the algorithms, the relevance of a document is determined by the number of used keywords that are included in the search query, to the total number of words in the text as a percentage.
For a query to be relevant, the frequency of its mentions in the text should be within 5%. If the number of mentions of keywords exceeds 7% - this can be assessed as overspam, if less than 3% - then such a text will be irrelevant due to under-optimization.