Client level files are files you want to share with your client, but do not belong a particular project. For example, you may wan to upload client information sheet that has information that is relevant to all projects.
To upload files at the client level, find a client record and click on the name. In the client Detail screen, there will be a tab labeled "Files from xxx" . The xxx is your company's short name.
Please keep in mind that files you upload to this tab are visible and downloadable by the client