Can someone please explain to me the voting topics?

Sorry for the lack of descriptive title- I didn't want to make it too long.

Basically... I'm not a huge reality TV viewer and only came back to Big Brother this series for the first time since about 2006-2007ish. I'd only ever voted in 2005, and that was pre-smartphone days and I used about a tenner in pay as you go credit as a 13 year old to get Nadia Almada to win... (Nadia, if you're reading this... I was a broke teenager and spent a fortune on votes and lip gloss to be a "Lip Gloss Bitch" so I'd happily accept a Superdrug voucher to compensate me for the lip gloss purchases).

So, I have never voted on an app, and I was surprised to find out that I could use 5 votes per eviction, all for free.

To get to the point... I'm hearing a lot of people in this sub and on Facebook talking about "vote to stay" versus "vote to evict" and how it would yield different results. Excuse my lack of understanding, but what does this actually mean, and why would these results be vastly different? Using a very obvious example of this series, someone who universally was agreed to be someone we all wanted gone (Kerry)... how would an outcome be different with both voting methods?

Basically... I'd very much appreciate it if some kind people on here could simplify and explain for someone like me what these voting scenarios mean, why they matter, etc...? So when I see it come up in future, I can be on the same page as everyone else.

Many thanks in advance, and if anyone wants a Kinder Bueno as a thank you gift, I would happily oblige 😊