It means internet service providers can hold both websites and their customers hostage.
Think about it this way. Comcast is your cable provider. Lets pretend Comcast buys out Blizzard Games (bad example, but I'm using well known examples). World of Warcraft is one of the most popular MMOs... so is Final Fantasy 14. Without net neutrality, Comcast can say to Square-Enix, "pay us eleventy-billion $ or we will throttle traffic to your FF14 servers". This could potentially drive traffic away from FF14 and towards WOW.
Again, this is a highly unlikely scenario; but its now something considered legal. It could possibly get even worse. Lets say, Comcast, instead of deciding to go after Square-Enix; goes right after the consumer. They can say "hey, if you want to play FF14 on our network, you need to pay us an extra $20 a month for our "gaming" package".
The supreme court can say all they want about net neutrality not being needed since there is enough market competition; but for a lot of people, they only have one source of broadband internet... their cable company. And cable companies are in fact, a state sanctioned monopoly. They cited google fiber as evidence of competition, and yet google fiber is only available in THREE cities.
