In the popular game Among Us this term refers to the mutual 100% accountability between 2 players that arises when both players have had 100% clear vision of each other for an entire round where 2/3 people have died.
This is usually a fairly hard line of evidence to use in meetings.
Soulbonded means 100% accountability, If two people agree that they are soulbonded, you can be sure their roles are in alignment, be it that they are both crewmates or an imposter duo.
*meeting called
White: "white and pink are soulbonded, we were together for the entire round and 3 people have died"
Pink : "yeah i can vouch for that me and white are soulbounded"
Blue: "ok so if either die now we can clear the other now, also very slight chance of white and pink being imposter duo"