Explain me like I'm 5: What is pit in safety car rule and how did it affect AD21

I started watching F1 in 2021 but since it was my first year I was irregular and did not had much idea about rules. AD 21 happened and I had not clue what happened. TBH, I still don't.

My lack of understanding comes from pit under safety car rule (i guess). Like, what is this rule? Why do some people pit but others do not? Ideally every one should. Please help a fellow F1 enjoyer out!