The actor took a shot of him reading a blurry page of text whilst in the makeup chair for the second season of The Witcher. Fans quickly unblurred the text and discovered that he was reading a plot summary of Mass Effect 3 from Wikipedia.
Several possibilities arise. The first and most likely is that Cavill is voicing a character in Mass Effect 5. BioWare recently confirmed that the game is in development, and Cavill as a video game fan in general and a known connoisseur of CRPGs would likely be up for a role.
The second is that a Mass Effect TV show or film is under discussion and Cavill is in talks for a role. Although a Mass Effect live-action project has been rumoured for a while, there hasn't been any firm announcements since a film proposal with Legendary Pictures and Warner Brothers petered out in the early 2010s. Although normally you'd expect a show or film to enter development and then talk to actors, sometimes the process goes the other way, so teams can go to studios with an actor with known star power already attached.
If that is the case, it'd be interesting to know whom Cavill would play. One would hope that a Mass Effect film or show would go with the games' gender-neutral casting, and allow both male and female actors to audition for the central role of Shepard (one alternative would be to ape the games by filming all scenes with both and allowing viewers to choose which version to watch, but that could be weird and expensive). Still, a Mass Effect film or TV series with Cavill as Shepard wouldn't be the worst thing in the world. I think I'd prefer him in another role though.
Whatever the case, hopefully we'll find out soon what's going on.