Here's a short video explaining what the 180 degree rule is for novice media producers. Along with the textbook descriptions and diagrams, this video will help you better understand the concept.
Watch this short lecture that offers a more in-depth look at the 180-degree rule. You will learn about how it came about and why it is important to maintain spatial continuity between shots.
In this video, you will learn about how to film a 3-person-dinner-table scene while maintaining screen directions. The same logic can be applied to any scenes with 4 and more characters.
In conjunction with the 180-degree rule, there is one more "rule" you need to always keep in mind. This video illustrates what the 30-degree rule is how you can apply it to your film/video-making.