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.
You can also use UML when investigating project functional requirements (UML Use Case Diagram) and for data flow and APIs UML Sequence Diagram.
Release Docs (UML v1.5 and UML v2.5.1) – https://www.omg.org/spec/UML
Recommended free UML software: https://www.diagrams.net/ (https://app.diagrams.net/ allow you to commit diagram directly to a github project)
Use case diagram: https://www.youtube.com/watch?v=OkC7HKtiZC0
Sequence diagram: https://www.youtube.com/watch?v=cxG-qWthxt4