Blizzard opened BlizzCon 2017 by announcing that StarCraft 2 would be going free-to-play on November 14th.
Everyone will gain access to the multiplayer and co-op aspects of the game. As for single player, StarCraft’s Wings of Liberty campaign will be free to everyone, and if you already have that campaign, then Blizzard is throwing in Heart of the Swarm for free. Otherwise, that expansion, as well as Legacy of the Void, must both be purchased separately.
So, to recap: Multiplayer ladder, as well as the co-op missions, will all be free, while 1/3rd or 2/3rds of the single player campaigns will be free depending on what you own.
You can read more about all this via the official announcement.