Auxiliary files are files that Studio generates in many circumstances as you edit and output your project. These are all stored under the disk folder specified in the image below. Click the file folder icon to change the auxiliary files' location. This is often done to save space on a particular drive.
The Delete button opens the Delete Auxiliary Files dialog, which lets you recover hard drive space by deleting files created while your project was being rendered. Auxiliary Files can also be deleted by selecting File -> Delete Auxiliary files within Studio.
If a problem occurs during the render process, it is best to delete the auxiliary files before rendering again.