You already have the ability to do this using the official Backup and Sync desktop software, but soon you'll be able to do it via a web browser too.
This new functionality is similar to that currently offered in Google Docs, Sheets and Slides and that offline editing has been around for ages. But now it's more powerful as it includes any file in Google Drive. Google says this includes PDFs, images, Microsoft Office files and more all via Google Drive in Chrome.
This is fantastic for anyone who needs to access files on the move while travelling, commuting or simply just away from a stable internet connection.
It is worth noting though, that the offline functionality of Google Drive will only be available via Google Chrome. You won't be able to use other browsers to access this setting. It's also not currently supported in ChromeOS, though the company says it is working on that too.