The Software Rendering Mode is specifically related to the Cinnamon edition of Linux Mint, and it is a mode in which you run the system when there's some sort of a problem with Cinnamon being able to directly access your video card, or acquire the resources it needs in order to run efficiently. When this occurs, you'll see a message after you log in, informing you that there was a problem and that Cinnamon has resorted to the Software Rendering Mode. This mode of Cinnamon gives you the opportunity to still be able to use your computer while you troubleshoot the underlying cause. Unfortunately, while the Software Rendering Mode gives you access to your programs and files, it's likely to be missing features you may be accustomed to. The following screenshot shows the Software Rendering Mode notification, which appears when there's a problem:

In most cases, issues that cause Cinnamon to resort to the Software Rendering Mode are typically due to drivers...