Zotero Storage Quota and Local Storage
Your Zotero Storage quota, displayed on your storage settings page, applies solely to files you've uploaded to Zotero servers, and doesn't affect your local usage of Zotero. You can always store as much as you wish locally — you'll never be prevented from saving to the Zotero app due to being at your online Zotero Storage quota.
If you're trying to free up space, it's important to remember that you might have far more files stored in your local Zotero library than your online storage quota allows. For example, you could have 5 GB of files stored locally, and your storage settings page would still show you as using 300 MB. If you then deleted 3 GB of files from your local library, emptied the trash, and synced, your storage settings online would still correctly show you as using 300 MB, because any files you deleted from the 300 MB would immediately be offset by additional local files getting uploaded and filling the remaining space. You would need to delete enough so that your total local usage was less than your online quota before you would see your usage drop below your quota, which in the above example would mean deleting 4.7 GB of local files.
If you'd prefer to keep all your files, you can add a storage subscription from the storage settings page to increase your quota. This would allow you to sync all your files and make them accessible from other devices and the web library, as well as to restore your files from the online library if something happened to your computer.