Honest Trailers are parody trailers of movies, tv shows, and video games. Like Cinema Sins, they often focus on exposing worn tropes and plot holes in humorous ways. They can also make up occasional fake plot lines by remixing clips from the film--for example, their trailer for The Matrix makes it look like the primary love entanglement is between Neo and Morpheus (rather than Neo and Trinity).
One of their most famous episodes was their trailer for Frozen. Personally, I preferred their trailer for Moana due to its better song parodies. I also really liked that they got Ryan Reynolds to appear as Deadpool making his own Honest Trailers for the two Deadpool movies.
As Screen Junkies been making trailers for nearly a decade, filmmakers have started taking to "Honest Trailer-proofing" their movies. Screen Junkies even has videos where they invite filmmakers to watch and respond to the Honest Trailers made from their movies. While certainly not all directors take this level of scrutiny into account, I do believe that its series like this and Cinema Sins that have led to the higher quality in the scripts and production value of recent movies. I feel that its an all-around win.