Net neutrality is about ensuring that your local internet Service Provider (ISP) treats all your internet traffic equally - not blocking or throttling it as they wish. Why would they want to do such thing? Mostly when they want to sell you a service and there is a free alternative out there, without net neutrality they can just block all access to it to users using their internet. It also enables them selling limited internet where you can only access certain things, which might not be terrible on individual level (as you chose to pay less) but is a bad thing on a bigger scale.
This particular vote only affects US residents and what dishonest practices their ISP can employ to get more cash out of them.
Net neutrality is not a universally enforced rule by US, it's regulated in each country individually (EU has some regulations about it too). Some countries don't have net neutrality at all. For example, a google search told me that Indonesia doesn't have net neutrality (https://www.eyerys.com/articles/news/indonesia-and-internet-neutrality-getting-away-it).
If you want to learn more in-depth about it- someone linked me a video about the history of this whole thing in US:
He starts going trough precedents of ISP trickery in US around 25 min mark.