At its most basic, file sharing is just what it sounds like: it's the practice of distributing or providing access to digital files, such as documents, multimedia (audio/video), graphics, computer programs, and e-books, among others.
There are different ways to share files. For example, you could use an email to send a file as an attachment. You might also use a USB drive to transfer a file from one computer to another. But in today's interconnected world, we often use internet-based services for file sharing.
Websites like Google Drive, Dropbox, and OneDrive allow you to upload files to a "cloud" storage system. From there, you can grant access to other people, either by sharing a link to the file or by inviting them to a shared folder. This makes it easy to share large files or collaborate with others on documents in real time, no matter where in the world you all are.