Visitors to dozens of major websites were seeing a special message about the future of the internet Wednesday, part of a broad campaign by the companies to stop what they say is a threat to the web as most consumers know it. Among the participants? Some of the biggest names on the web, including Facebook, Google, reddit, Twitter and Spotify.

In dispute are a set of federal regulations saying that internet providers should not slow down, block or charge websites extra fees while treating other sites differently. The Federal Communications Commission is currently weighing how to repeal those so-called net neutrality rules, in a move that’s expected to benefit internet providers such as Verizon and AT&T.

Tech trade groups and activist organizations, such as the Internet Association and Fight for the Future, argue that repealing the FCC’s net neutrality rules would give internet service providers too much power to determine what consumers can and can’t see online, and for what price. So they’ve partnered not only with the likes of Google and Facebook but also some of the world’s other top online destinations – including Airbnb and Netflix – to oppose the FCC’s proposed actions. Here’s everything you should know about Wednesday’s “day of action”:

What are we seeing on these websites?

It’s up to each site to decide how far to go but many of the participating businesses are publishing blog posts, adding a banner to their homepages or even changing their user interfaces in ways that make it impossible to browse those sites without learning about the issue of net neutrality. Some, like reddit, have created a slow-loading pop-up message in an attempt to show what a world without the FCC regulations could look like.

Others, such as Netflix, have placed a banner at the top of their homepages drawing attention to the issue.

Even Pornhub, one of the internet’s most popular porn sites and the 18th-largest website in the country, is on board.

So what is net neutrality?

Net neutrality is a principle about fairness on the internet. It holds that no ISP should be able to unfairly manipulate your internet usage or your experience of the web, particularly in ways that harm other businesses. Proponents of the FCC’s net neutrality rules, which were passed in 2015, say strong regulations are necessary to prevent ISPs from artificially directing customers to sites and applications they control or with whom they share special business relationships. Opponents say the rules are overly burdensome and that softening the rules would help them upgrade their networks.

The rules also ban internet providers from extracting payments from websites as a condition of having their content delivered to consumers’ screens. The rules apply equally to wired and wireless providers, and also allow the FCC to investigate carrier practices that it deems are a potential threat to competition.

The FCC wants to repeal its own rules that were passed two years ago?

That’s correct. When President Trump was inaugurated, the FCC passed from Democratic control to Republican. Now, currently with a 2-1 majority, the Republican FCC is seeking to roll back the net neutrality regulations.

Why do internet providers oppose the FCC rules?

Not all Internet providers are against the rules. For instance, some smaller providers are supportive of the regulations.

“Internet providers should not be able to charge content creators – like Netflix, Hulu, or CNET – more money to stream their service, or have the ability to block others entirely,” said Dane Jasper, chief executive of the California-based Internet provider Sonic.

But more broadly, the industry has argued that the net neutrality rules prevent it from finding new ways of making money. At a time when many internet providers are looking to diversify their revenue by going head-to-head with Google and Facebook’s advertising prowess, it’s no longer enough simply to provide an on-ramp to the internet, analysts say.