This is a very useful query but one that should be used with caution, you can see why from the title. This query will delete data in a table and this cannot be undone. So as always make sure you have a backup of your data and if you are using this for the first time or need to practise then do this on a copy.
The delete query is useful for deleting data you no longer require, this could be because it is out of data and no longer needed or because it has already been archived. You can archive specific data from a table using the make table query, you can see how to use the make table query by clicking here.