Keys, fills, kickers, and background lights—what exactly do each of them do?
As a beginner, lighting can be really intimidating. Not only do you have to learn how to properly handle those big, scary studio lights (if that is in fact what you're going to be working with), but you have to understand what each individual light in a lighting setup actually does, from the key to the kicker. To help with this, Mark Wallace from Adorama helps demystify a few of these lighting basics by demonstrating how keys, fills, kickers, and background lights affect the look of a subject.
Understand that the video is geared toward studio photography and stuff you'd typically see in fashion videography and commercial work, but the same principles can still apply to narrative and documentary filmmaking. In fact, the lighting setup you saw demonstrated is one of the first taught in film school.
But the most interesting and important lesson Wallace tries to teach, in my opinion, is how different lights work and how they affect your subject individually. Being able to see a subject lit with just a key or just a kicker or just a fill can help you gain a better understanding of how light behaves depending on its intensity, shape, and placement. This will then not only help you make better stylistic and creative decisions when you light your own scenes, but it'll help you solve problems when they arise, like, for example, lighting a scene with limited sources.