Collections

I don't think I really get collections yet. I know how they work, and I've watched a few videos that tried to give examples, but I still feel like I don't know what they're actually about.

When I try to explain it to myself, I get something like "collection is what you call any random page that you are going to index and/or link with other pages". There is probably more to it though and I'm probably not getting all the implications.

So. What am I missing? Are there any good explanations? How would you explain what collections are or are for? How do you personally use collections? What are your routines around them?