Used for generating SQL clauses that filter a primary query according to object taxonomy terms.
WP_Tax_Query is a helper that allows primary query classes, such as WP_Query, to filter their results by object metadata, by generating
WHERE subclauses to be attached to the primary SQL query string.
A class is an entity that determines how an object will behave and what the object will contain. In other words, it is a blueprint or a set of instruction to build a specific type of object. The files of WordPress define many useful PHP Classes. This Site will try to list all/most of the core classes.