Google Query Language is the part of the Google Visualization API that lets the developer perform various data manipulations with the query to the data source. This could also be used in the Google Sheets QUERY built-in function.
Google Query Language allow developers to perform various data manipulations using a syntax very similar to the SQL syntax. It's part of the google-visualisation and can be used in google-sheets through the QUERY built-in function.
Before posting a question about the Google Query Language check the reference documentation.