Extensions are third-party operating system kernel extensions, that connect into the most protected parts of macOS itself, with full access to everything in macOS and everything you have stored on your Mac. Various of these extensions have been a source of crashes, as well as risks of exposing your data for whatever the developer might (or might not have) intended. Apple has been locking down these extensions for a while, and is now replacing this mechanism with one that isolates access, and reduces the chances of crashes.