As @asolanki mentioned, Kafka will automatically failover leaders to brokers that are alive. I just wanted to note that this is not called a reassignment - it's called a leader failover.

In Kafka, a partition is assigned to N number of replicas (N=replication factor). The in-sync replica set (ISR) is the set of replicas that are in-sync (alive and have the latest data). The ISR naturally follows the replica set.

A reassignment is when you change the replicas that this partition should be hosted on. I think an example would make this clear:

A simple failover:

1. partition=0 is on brokers (replicas) replicas=[1,2,3], isr=[1,2,3]. Broker 1 is the leader
2. broker 1 fails. Kafka automatically fails over to another broker
3. replicas=[1,2,3], isr=[2,3] (broker 1 is dead)

A simple reassignment:

1. partition=0 is on brokers (replicas) replicas=[1,2,3], isr=[1,2,3]
2. We reassign partition=0 to replicas=[4,5,6]
3. replicas=[4,5,6], isr=[4,5,6]