It's a system of staking accounts for Splinterlands. Card owners delegate to our accounts, staked players play them, and the earnings are split automatically. Best intro post is probably this one:

You can't send DEC to a guild from one of our accounts without the active key, so if you wanted to for some reason you'd have to ask me to do it.

As a card owner you're welcome to have your account be in one of your guilds rather than ours. Contributing from it would be awkward, though.