I’ve installed plex but I don’t even know where to start with it. Is there an “idiots guide” anywhere that I can follow?

What can it do? Can I access it from my chromecast? Is there a difference between plex player and plex? Why do I even need it? If I get it to work, what else do I need to make it useful?

So many questions and most guides I’ve found are far too advanced, or they don’t tell you enough. I want to know, just explained in a way that doesn’t assume I’m an plex expert.