1. Hinted Handoffs - if a node has a transient failure, the other nodes store up messages, like your buddy might take notes in class if you had to go to the bathroom. They'd pass you those notes when you got back. "Here's what you missed." When the node comes back online it processes all new operations and works through its backlog of hinted handoffs to get caught up. Because of the backlog it creates, hinted handoffs are only stacked up for a few hours. If the node never comes back up, or comes back after that window...

2. Repairs - in an eventually-consistent database you might miss an update or two over time. Or maybe you're a replacement node that has to fill in for a failed node. The replacement will get streamed data from the other replicas to get it started, or you might restore sstables from a backup, but then you should run a repair job to make sure all your replicas are properly in sync.

(That's my understanding. Let me know if that sounds correct from the hands-on experts.)