A redeeming characteristic is one that counteracts or corrects something negative. If you can only think of one good thing about your next door neighbor, for example, that one thing is her redeeming quality.
Your grandpa may be a difficult person who constantly criticizes you and complains about life. If, however, he makes the best chocolate chip cookies you've ever tasted, you can say that cookie baking is his redeeming quality. And if your dog is truly odd-looking, not cute at all except for her long ears, those ears are her one redeeming physical trait — they're the positive thing that balances out all the negatives.