This new feature is the XML-aware counterpart to PhpStorm built-in functions Find in Path and Search Structurally: It allows you to find occurrences of certain XPath expressions in all XML files in a specific scope. It is available as Find by XPath... under the Search menu.
The scope that can be chosen is either the full project scope, a specific module or a simple directory. There's also the ability to use custom scopes, but this is somewhat limited as it will only scan XML files inside source-folders.
This dialog provides the same functionality regarding completion, error checking and intentions as described in Evaluate Expression.