It enables for example "Space/Shift dual role key." When you press the Space key alone, it's a space; but when you press it with another key, it's a shift. E.g., space + x = X, roughly speaking.
Any pairs of keys are possible. For example you can turn your Left Alt into BS/Alt in addition to Space/Shift. In this example, Space followed by LAlt sends Shift + BS, LAlt followed by Space does Alt + Space, and Space + LAlt + x and LAlt + Space + x is Alt + Shift + x.
It's a fork of Xorg "evdev" driver = xf86-input-evdev. It's only for Linux - X in other platforms don't use evdev driver. You can use AutoHotKey for MS Win and Karabiner (formerly named Keyremap4macbook) for Mac instead.
With this hack, your hands can stay at the home position almost always, and feel more "at home", thus "At Home Modifier".