From the spec:

All drivers that implement a connection pool MUST provide an API that allows users to subscribe to events emitted from the pool. If a user subscribes to Connection Monitoring events, these events MUST be emitted when specified in “Connection Pool Behaviors”. Events SHOULD be created and subscribed to in a manner idiomatic to their language and driver.

For a precise description of each individual event, see:
https://github.com/mongodb/specifications/blob/master/source/connection-monitoring-and-pooling/connection-monitoring-and-pooling.rst#connection-pool-monitoring-1