Every file and folder within your hosting plan has a set of permissions. If these permissions are misconfigured, your website might not work as expected.
To quickly fix all your file permissions, go to Websites → Manage, search for Fix File Ownership on the sidebar and click on it:

Next, mark the checkbox to confirm that you want to set the files and folders to default permissions and click on Execute:

Fixing file ownership will set the permissions to the followig values:
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For files, the default setting is 644
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For folders, the default setting is 755
If you need to configure specific permission settings for files, you can easily do so using the File Manager.