Consistency levels in Cassandra can be set on any read or write query. This allows application developers to tune consistency on a per-query basis depending on their requirements for response time versus data accuracy. Cassandra offers a number of consistency levels for both reads and writes.
You should first understand about QUOROM
QUORUM is a good middle-ground ensuring strong consistency, yet still tolerating some level of failure.
A quorum is calculated as (rounded down to a whole number):
(replication_factor / 2) + 1
For example, with a replication factor of 3, a quorum is 2 (can tolerate 1 replica down). With a replication factor of 6, a quorum is 4 (can tolerate 2 replicas down).
For your question the explanation is given below
(nodes_read + nodes_written) > replication_factor
R + W > N
For example, if your application is using the QUORUM consistency level for both write and read operations and you are using a replication factor of 3, then this ensures that 2 nodes are always written and 2 nodes are always read. The combination of nodes written and read (4) being greater than the replication factor (3) ensures strong read consistency
You can read more about Quorom and consistency in detail in the link posted below