Very simply put. Splunk is a logging tool that takes up to millions and multiples of millions of logs from anywhere, organizing them and giving a readable interface to view that data. These logs can be from system event logs, or actual traffic logs, to line upon line of communication on servers, network devices, and other endpoints, firewalls etc. Splunk is built to be a 1 stop resource for an entire network if it is configured to do that. Can go more technical if wanted as well.