Kaldheim is both a plane (self contained world in the MTG universe) and the name of the first Magic expansion in 2021.
Kaldheim is a Nordic themed world. So you can expect Magic’s take on Norse mythology, vikings and other similar tropes.
We first heard about Kaldheim back in 2009. It was featured on a Plane card (from the Planechase set).
Just like Zendikar Rising and Commander Legends, Kaldheim releases with two associated Commander decks. Each deck has 8 new cards (that's more than ZNR and CMR!) and comes with 100 cards (1 foil commander and 99 non-foil cards), 10 double-sided tokens, and 1 Life Wheel™. These decks are a great way to get into the format or just try something new. 
The pictured commanders are in seperate decks.