When importing video and audio into Adobe Premiere Pro, it processes versions of these files that it can readily access for faster performance. These are referred to as "media cache" files. These are stored in the Media Cache Files folder. In addition, there is a folder containing a database, which retains links to each of the media cache files. These are referred to as "media cache database" files. They are located in the Media Cache folder.
Note: there is a delay in proper audio and video playback while newly imported media is being processed and cached. A progress bar in the lower right of the application shows the progress of the media cache being processed.
Sometimes, these cache folders get too large, or contain a corrupt file, so you need to know where to locate them for deletion. By default, both the cache files (in the Media Cache Files folder) and the media cache database files (in the Media Cache folder) are stored in these locations:
Mac: /Users//Library/Application Support/Adobe/Common
NOTE: By default, AppData is a hidden folder. Check this tutorial on how to access it: How to Find the AppData Folder in Windows 10 - YouTube
You can change the media cache location under Preferences > Media in Premiere Pro
In order to clean (or "trash") the media cache, you can delete these files. With Premiere Pro closed, navigate to the location of these folders and delete the files manually. After you delete the files, then empty the trash.
Note: Deleting the files manually with Premiere Pro closed deletes the current media cache files, as well, which is what you want to do when troubleshooting errors.
There is a 'Clean' option in Preferences > Media too, but that only deletes cache files associated with files that are not currently being used.
After deleting the files, you should see additional hard drive space become available, as the media cache files can take up significant space. If you have older projects you have completed, it is a good idea to delete these files to save storage space and to keep your computer's hard drive tidier.
Keep in mind that a delay also occurs after you delete all cache files and then open a current project as the files need time to rebuild. If you have a large project, budget enough time for the media cache files to rebuild. This can be done overnight, for example.