You might want to read Richard Byrne’s post about it to learn more, but it basically allows you to take historical clips and make your own “remix” of them.
I have to say I have questions about how much value is added to student learning by the remixing part of the site (at least for English Language Learners) — it seems to me more trouble than it’s worth (though the site does seem to try to make it as easy as possible).
However, the site does have a good collection of useful videos. If Remix America isn’t blocked by school district content filters, those short videos could be very useful. And you don’t have to register or sign-in to view them. You only have to register if you want to create remixes.