Google Drive is a file storage system that is cloud based and can be accessed via your Gmail account Google Drive allows users to store files on their servers, synchronize files across devices, and share files. Files created and edited through the google office suite are saved in Google Drive.
Google Drive Security Update - You probably received an email from Google Drive Team saying that "A security update will be applied to Drive."
What does this mean?
It just means that documents that are not native to Google Drive, (i.e. PDFs, Excel, etc) and that are shared will be affected. If someone is unable to view your file after the security update, it will notify you by email that someone is requesting access.
People who previously viewed the file will not be affected, but anyone who hasn't opened it will need to request access again.