Great question. Focus chests are from the Daily Focus. The Daily Focus used to be the daily quest. Except now, the more you play in a day the more daily chests you can earn.

See the right hand red arrows. My current focus is Fire splinter. I haven't played any today so I have not earned toward the chest. Depending on what cards I play that I own and the types of cards, I earn points toward the focus chest. I usually average 60 points per win, but with a good streak, I might receive 150 points per win. In 24 hours, I can generate 5 to 10 focus chests in about 1 hour of playing time. Focus chests are available each day to be opened.

In addition to focus chests, the daily focus also earns points toward the season chests. Season chests can't be opened until the end of the season, I think. See the left hand side with red arrows. Those are my season chests, and all wins, including the focus wins count toward season chests. Currently, I have 36 chests for the season.