Each node holds 50% of data on 2 nodes with replication_factor=1
Replication Factor 1. Assuming that the cluster is balanced. There is 1 copy of your data (1 replica) in the cluster. Since you have 2 nodes and one replica, then each node will have 1/2 of the data of half a replica each. When you shut down 1 node, you lose access to 1/2 of the data.

Replication factor 2. There are 2 replicas of the data in the cluster. 2 nodes and 2 replicas -> each node can have 1 full replica. When you shut down 1 node, the other node still has a complete replica.