"yml" is the common filename suffix for files in the YAML (YAML Ain't Markup Language) format.
YAML is a structured notation that supports hierarchical data and collections. Its popular relatives include XML and JSON.
Because of this simplicity, the amount of data required to transmit over a network or store in a file is very small. Even better, it's very easy to edit YAML, as you don't have to fiddle around with angle brackets, braces, and other structural stuff. As in PYTHON, most of the structural information is conveyed by simple indentation.
A very quick look at the project you pointed to seems to have only one file (my eyes aren't what they used to be, though) and that's a Travis configuration File. I believe that Travis is a test framework, so to be complete you should keep that file even if you never run Travis yourself.
Yes, you really should learn YAML. Basic YAML is quite simple, but very powerful and you can expect to run across a lot of it these days.
The main page for the YAML project is an outline that is itself in YAML form: https://yaml.org/
Note that it lists several Java YAML tools. Just about every major and some minor programming languages have YAML support.
A description of how YAML is constructed (the YAML spec) is here: https://yaml.org/spec/1.2/spec.html