It's rather complex to explain what dotnet is and does. It should, and probably was, installed at the time that Windows 8.1 was installed.
When I upgraded Windows 8 to Windows 8.1, at some stage near the end of the upgrade, I saw dotnet being installed on my computer. I was never asked if I wanted it installed, it just installed it (I would have allowed it to be installed even if it had asked anyway).

If you want to know if it's already installed, go to the Control Panel > Programs > click on Uninstall a program > Scroll down the list looking for Microsoft .NET Framework 4 (you might actually find that you have v4.5 or better).