Every ISP is part of the mix, including the "upper level" ones who don't deal with "end users" like you and me, but only provide the big pipes between the "smaller" providers.
Net Neutrality says that every packet trying to get from point A to point B is treated the same -- they all get in the same line, and get the same treatment along the way -- kinda like when your bank has one line for all customers, and each one gets handled by the next free teller (and all the tellers can handle all transactions).
Breaking Neutrality means that the bank customer who has a bigger account, or handles more transactions there, or just plain pays the bank more for the privilege, gets another line, and maybe a special teller who doesn't handle the other customers, and whenever the fancy customer shows up, they go ahead of the main line. And higher bidders may get more new lines and/or tellers, and so on, and so on.
If you can't pay extra, you may wind up waiting forever to get a teller to handle your business -- or to get your packet from A to B.
If people have to wait forever (hours, or even minutes) to get the packets with their favorite indie band's music coming from TheNewMusicSite (who isn't paying the ISPs extra because they're new), but the packets with music coming from YouTube or iTunes or Amazon arrive in seconds (or less) ... they're less likely to listen to the indie band (who may not want to put their stuff on Amazon or whatever, for perfectly valid reasons).
Imagine if paying more got you a private lane on the highway at rush hour -- and every time someone else bid higher, the lanes shifted, so only the highest bidder was driving along at highway speeds, and everyone else was in traffic matching the degree to which they bid lower... down to the poorest people who couldn't pay any extra, who are always crawling in permanent LA rush hour traffic...
That's the Internet future we're looking at, if Net Neutrality dies. Not that the basic subscriber's data will *never* get there, just that it'll take (some, maybe much) longer (Of course, the EU just made NN law, so this might just be another reason to figure out how to move overseas... or subscribe to an EU ISP who decides to break into the US markets with this extra special value-add...)