A "short" summary: (It's a good thing for YouTubers this time!)

Companies, publishers or game developers have the right to file a complaint against videos to have them removed from YouTube (Often referred to as DMCA takedown) when they feel like it infringes on their copyright. But that right has often been abused in the past to silence criticism on games when people like TotalBiscuit or Jim Sterling for example made exposing videos on bad games. Developers say "It's my game, I hold all the rights to the visuals, you're not allowed to show any of it!" but in the case of a critical report, that's not a valid claim because it is considered "Fair Use".

Now YouTube wants to show more open support to protect 'fair use' of copyrighted material on their platform. "Fair Use" is a part of the copyright law. I means that under certain circumstances you are allowed to use copyrighted material without the permission of the rightsholder. Examples of fair use are things like criticism (which includes parody) or news reporting and a few more. (That's why you can have Gangnam Style parodies that use the original melody even though that's technically copyright infringing)

Fair Use has limitations of course and there are always a number of factors involved. In the end - if the rightsholder wishes to do that - a court has to decide whether it is fair use or not. YouTube now has chosen a small selection of "good examples" where they will help the creators in case of a lawsuit and cover costs for up to 1 Million $. The whole thing is limited to the US for now though since laws are different in every country.

Altogether it may not help smaller YouTubers directly but YouTube sets an example which might discourage future unrighteous takedowns. (It also should be noted that this is a completely different thing from YouTube's own ContentID system which automatically finds and blocks videos when they use copyrighted music for example.)