The Global Groove settings are found at the end of the effect chain in the Master Track and allow you to shift the playback of the pattern lines in a beat back and forth, creating a swinging feel for the whole song.
Adding groove with a slider means that the first note will be extended, while the second note will be shortened and triggered after a delay. Note that the sliders will apply to a different amount of pattern lines when using different Lines Per Beat settings.
For example: Increasing the 0&1 slider and setting other sliders to 0 will make the first pattern line longer, delaying the second pattern line, repeating after 8 lines (at LPB 4). Setting all sliders to ~50% will delay all second, fourth, sixth and eighth lines, resulting into a typical swing groove.
It's not vitally important to understand the exact technical details here, since groove is about adding a feeling to the beats. The best way to get a feel for the settings is to just play around with them. To make it easier to hear what's going on when you do, try adding something like a steady hi-hat line, which will be triggered fast and often.