
That is the user ID number for the different accounts. Depending on how many accounts you have set up in your Steam client, you will see one or more folders with a number in place of its name. In your Steam directory, you should find a folder labelled "userdata".

If you've installed Steam somewhere else, you'll need to navigate to the Steam folder wherever you installed it. On a Linux system, the default is ~/.local/share/Steam. On a Mac, the default is Users//Library/Application Support/Steam, with username being the Mac username, not the Steam username. On Windows 8 and Windows 10, the default is C:\Program Files (x86)\Steam. Finding and opening Steam’s installation directory depends on the operating system you’re using.
