As soon as you create, join or interact with a card, it's yours to keep. We've introduced keeping as a way to make sharing fairer. A friend might share 100s of cards with you but only a few of them might be relevant to you, so only the ones that are interesting to you, you keep and count towards your card count if you are on the free plan.
All the cards you author are automatically kept. If a card has been shared with you, if you do any of the following the card will be automatically kept:
Adding the card to a parent
Adding a tag
Adding a child card
Note: commenting is excluded – so you can offer advice and help even if the
card might not be directly relevant to you.
If a card has been shared with you, you can see if you've kept it, by looking at the member list, as shown in the gif below:
All the cards you have kept will be available in your Home, even if they get cut from the original parent card you found them in or if your permissions change. The number of cards you have keep is equivalent to the number of cards contributing towards your card count.
You cannot 'unkeep' a card, however you may junk it and it will no longer contribute towards your card count.