Cannot wrap my brain around pruning

Hey all,

I'm sure this question has been asked before but I can't find any answer that explains it well enough to me.

I am trying to understand Pruning. I do not understand the "Keep 1 snapshot every x days if older than y days"

I'm trying to figure out the best backup period, I think I've settled on a weekly backup (every Sunday night). If I go with a backup once a week, how can I set my pruning up so I don't just have a ton of snapshots. Ideally I'd like to keep only the past 3 weeks of snapshots

Thanks for any help!