Event Time and Watermarks can u explain it : I am new to flink.

and trying to learn the Event Time and Watermarks section (without success so far).

https://ci.apache.org/projects/flink/flink-docs-release-1.4/dev/event_time.html#event-time-and-watermarks

can you explain what is it watermarks, what is solves?
the example is not clear to me

thanks.

