DSL is a “Domain Specific Language”, in this case the “native” rules language developed for openHAB version 1 and 2. It’s based on Xtend which in turn is based on Java.
Because of that history, a great many examples, most of the older forum postings, and most of the docs refer to it.
Because of that history, it continues to be available in OH3, though not really viewed as primary rules language.
javascript is javascript, a lot of general resource across the web.
In the openHAB environment, you’ll also see reference to JSR223 rules which generally means javascript.
For this, you could do worse than start at -
which was written in the context of using javascript rules in OH2 (hence “new rules engine” references) but is still sound for OH3.