The ‘Wikidata Custom Query’ tasks is used to access the very rich and growing data universe of wikidata.
The query is executed as a SPARQL Query. There are plenty of good sources on the internet to learn how to use this language, e.g. https://www.mediawiki.org/wiki/File:Wikidata_Sparql_Query_Tutorial.webm .
The task is very powerful to automize mapping of data. If you e.g. need to map your company car number-plates to their cities, you can fetch the mapping table via a wikidata-query and map this table with a JOIN task to your data.
Go to Flow Gallery and Copy & Edit the Wikidata Flow for example usecases.