Every developer should have a basic understanding of UML 2.0 Class Diagram. It is used to represent class structure and interaction with other elements in a diagram. You can also find it in technical documentation there fore it is a must to know how its written and how to read those diagrams.
Release Docs (UML v1.5 and UML v2.5.1) – https://www.omg.org/spec/UML
Use case diagram: https://www.youtube.com/watch?v=OkC7HKtiZC0
Sequence diagram: https://www.youtube.com/watch?v=cxG-qWthxt4