File hosting services let you mirror a directory on your computer to the cloud (internet). This is useful for backup and for sharing files across computers.
There are a wide variety of such services available. The most popular is probably Google Drive. Like most Google products, Google Drive is absolute junk. It has a horrible user interface, doesn't work reliably, and I can't figure out how to use many of features. Given how horrible it is, it is unsurprising that it is so popular with modern morons. I have also looked at many other file hosting services and they are generally bad. By far the best is Dropbox. It has a clean user interface, is reliable, and has many useful feature like file versioning in the paid offering of the service.