Someone out there owes the character a favour. It can be anything from a sum of money, repayment for introducing them to their future wife or even saving their life. Whatever it was, they’re not going to forget it, and in the character’s moment of need they can be called upon to help out in a dire situation.
As a Minor Trait, the favour is something relatively small – a small amount of money (only £1000 or so), introducing someone to them at a party who later became a valuable business client or romantic partner, or you bailed them out when they were in trouble.
As a Major Trait, the favour is more important, from a large amount of money (over £10,000) to saving their life. If appropriate, you may opt to call in the favour and seek assistance in the current adventure. Of course, if you ask for a favour bigger than the original debt, they may walk off afterwards saying “we’re quits, don’t call on me again”, or you could even end up owing them a favour, and gain the Owes Favour Bad Trait.