Here’s the official explanation of the new modes:
Once players download the free update and complete the main story, Balloon World will become available, and players just have to find Luigi in each Kingdom and speak with him to get started. In “Hide It” mode, players have a limited amount of time to hide a balloon, while in “Find It” mode, players have the same period of time to locate balloons hidden by other players from around the world. Creative use of captures is key to success! Whether players are hiding balloons or finding them, their Rank will go up based on their high score. While playing Balloon World, the atmosphere will change for each kingdom: for example, it might be raining in New Donk City, or the sun might be setting in Tostarena.”
Along with this, a range of new outfits will also become available, including the Sunshine, Musician and Knight Armor. Snapshot Mode will also be getting some fun new filters Neon and Coin.