When you're new to lighting, the veritable plethora of principles, terms, and techniques can be a bit overwhelming, but there are a few fundamental principles that you should absolutely learn to ensure your understanding is secure. This great video talks about one of those principles.
Coming to you from Aputure, this helpful video talks about the inverse square law and its effect on light falloff. One small correction: while the video says light intensity is equal to the reciprocal of the square of the distance, it actually means it's proportional to it. It's particularly important to understand this, as humans are in general absolutely terrible at intuitively estimating nonlinear phenomena such as this, and if you rely on your intuition when lighting a set, you might be thrown off by the behavior of the light. It's important to actively think through the lighting instead of relying on intuition. The more you practice lighting while keeping this principle in mind, the more you'll get used to it and be able to overcome the inaccurate heuristics your brain would normally rely on. Check out the video above for the full rundown and some great examples of the principle in action.