Lighting is one of if not THE most important part of any film/video. It doesn't matter how good the camera is or how good the makeup is or the performances, if the lighting is terrible, it will look terrible.
In this video posted by thesubstream, and made by The Film Lab, gives a very clear and easy to digest tutorial on lighting ratios.
You could of course just play around with lighting until you get what you like, but lighting ratios are a way of dissecting it down to repeatable/explainable results.
A ratio in lighting basically explains the relationship between light A and light B, if light A is twice as bright as light B, then there is a 1:2 ratio. Simple, right? Watch the video for the rest!