TYPO3 stores processed files and cached images in a dedicated directory. This directory is likely to grow quickly.

With this action you can delete the files in this folder. Afterwards, you should also clear the cache database tables.

The File Abstraction Layer additionally stores a database record for every file it needs to process. (e.g. image thumbnails). In case you modified some graphics settings (All Configuration [GFX]) and you need all processed files to get regenerated, you can use this tool to remove the "_processed_" ones.