Event correlation is the process of taking data from multiple dataset and types and associating them together, in order to get an overall view of the system, instead of view each event in isolation.
Imagine you have a web application that suddenly slows down, and you need to figure out why. You have logs, traces and metrics for your web application. You can correlate the logs, metrics and traces to piece together the clues from each data type. The metrics will tell you there was a sudden increase in CPU usage of the database, the traces will show how all the requests that were slowed down in the database, and the logs might highlight that you're trying to retrieve too much data at once from the database.