A Datacenter in Cassandra is basically a group of nodes, within a cluster, that are related and configured for replication purposes. This architecture is followed in Cassandra, to prevent Data loss, and help speed up the processing. The related nodes within Cassandra, are configured and set up to reduce the latency, prevent transactions from impact by other workloads and other related effects.
If you are a novice to Cassandra and wish to learn and get trained in it, then check out the Cassandra certification course from Intellipaat. Also, do give a watch to the following YouTube video on Apache Cassandra Training.