Relevance tuning allows you to to give a boost to a result in the response when the query includes terms that match the attribute. For example, you might have two similar documents but one is created more recently. You might want to boost the relevance for the newer (or older) document.
In order to allow an attribute to be used to boost a document you need to mark it as searchable.
This is what the Relevance tuning on the Search console looks like:
Go back to the Facet definition screen and mark _category searchable.
On the search console, check what attributes do you have available for relevance tuning.
A typical example for boosting is the case of when a document was created (using the field created_at):
Try a query adjusting the freshness boost value for _created_at