This is the second video in how to set up Moodle groups, please take a look at #139 Moodle Groups [1/2]: Setting up your groups first.
Groupings are collections of Moodle groups. Students can be members of multiple groups, and groups can be members of multiple groupings. For example, you might want to create a grouping that contains just the tutor groups you are responsible for, or you might want to create a grouping for all the student working groups.
Generally speaking (but not in all cases) it is the grouping, and not the individual groups, that you assign to content in Moodle.
For more help on how to set up your groupings, see the video below: