A Decentralized Autonomous Corporation (DAC) is the equivalent of a guild in Star Atlas. The term was first coined by the team in their White Paper, and is a play on DAO (Decentralized Autonomous Organisation). Since this game allows you to earn money, meaning (the members of) guilds earn money, they switched out the "O" for a "C" to emphasize this aspect.
Besides being called 'DAC', the terms guild, corporation, (player) organization and clan are also often used.
Despite the fancy term, there is at the moment nothing decentralized about a DAC from the get-go, let alone it being autonomous. The team has repeatedly stated that they will not provide any tooling to this end for the time being, though they do not rule out they might facilitate this in the future.
With support from the team lacking, some DAC's are (thinking about) setting up their own DAO internally.
Do I need to be in a DAC? No, you can play the game without being in one. However, the universe is a big place and the team has often suggested it might be a good idea to team up.
Additionally, this game continues 24/7, meaning your assets in the Medium-, & High-Risk zones will be unguarded when you are asleep.