“What is net neutrality and why is it so important to maintain?”—
The basic principle of Net Neutrality is that access to all websites and web services should be equal and that anyone can start their own website/service and make it accessible to anyone with internet access, just like any other website/service.
Without Net Neutrality, your Internet Service Provider (e.g. BT) could arbitrarily block whatever websites it didn’t want you to access (e.g. perhaps blocking you from accessing competitors’ websites). It could also mean your service provider purposefully degrades access to certain websites or services and/or forces you to pay extra to access certain websites or services (imagine paying an extra $5 a month to your service provider just because you want to access reddit).
In the UK, this is not as big of a problem because there is (for the most part) much more competition because the BT network is opened up through a wholeale program which allows third-party providers to access the network and provide their own differentiated/custom internet services (to some extent).
In the US, however, there isn’t really competition like this. Many homes have only one broadband internet service provider to choose from (e.g. their local telephone company) and some people are luckily enough to have two providers (e.g. telephone and cable company) or in very rare cases three providers (e.g. telephone, cable and independent fiber/fibre company). Basically though, there is hardly any competition which means that if one service provider starts violating the principles of Net Neutrality, many customers are completely powerless (since they have such a limited or perhaps no alternative selection for ISPs).