It means there are five unread notifications about this card.
You can read more at https://help.trello.com/article/793-receiving-trello-notifications - there's even an animated gif there that shows a red bell appearing and disappearing as you read them!
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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