PPLNS for dummies? Rounds, shares and time - I'm so confused!

Don't know if I need to clarify that I am mining for XMR. Sorry if this has been asked before, I couldn't find it by searching.

The FAQ shows a picture of shares rounds and I simply can't wrap my mind around the text written for it. Rounds 1 and 3 something. Rounds 2 and 4 something else. Difficulty * 2 something. I'm by no means a stupid person but seriously, can someone please dumb it down for me and explain it like I'm five?

What defines a round? How long is a round? How do I know which round is when?

In another topic I saw someone mention they had approx. 600k shares, someone said they had 450 t shares. 600k I assume meaning 600,000. 450 t meaning 450 thousand (450,000)?

"Shares" seem to correspond with my miner's "pool-side hashes", more precisely "total number of pool-side hashes since last reset". Please correct me if I'm understanding this wrong.

I don't have a monster rig for mining, but I know I had a higher number of shares (as defined above) after just a few days of mining since I found this pool. I guess what I'm wondering the most is this: how do my millions of shares from a couple of days of mining stack up to the 600k/450k (as I've understood above, again please correct me if I'm wrong) accumulated over the period of a month or more?