As the names suggests this type of action query is going to delete data so if you are doing this for the first time try this on some data you are not worried about losing or on a copy of your data. You should always have a copy of your data particularly if you are going to use queries like the delete query. So before you do anything please make a backup of your data.
So you may need to delete data for data you no longer need or because you have already archived it and you can now remove it from the table. If you want to know about creating an archive you should take a look at the make table query I have here.