In the website context this means sites that are designed to be viewed on devices with lower resolution screens than desktops.
Other characteristics of users with mobile devices:
- They may not be using a mouse, but a touch screen.
- They may have very specific aims when visiting a site, such as finding directions.
- They may just be killing time.
Note that some mobile devices have screen resolutions approaching desktop resolution, so may not want a "mobile" version of a particular site.