Since your question is phrased so broadly and there are tons of research on Universal Grammar (UG), I have to write a likewise broad answer, which is nevertheless technically correct:
Universal Grammar is the genetic component of the language faculty (Berwick and Chomsky 2016).
Bernard Comrie (as early as Comrie 1989) noted that this research paradigm - whatever it is called now (is it the Minimalist Program or biolinguistics?) - is aprioristic, which is legitimate. cf. "Conversely, one can formulate a theory of UG as an abstract Platonic object with no claim whatsoever regarding any physical instantiation it may have" (Roberts 2017).
However, it means it could be the case it is not "potentially disconfirmable" (Comrie 1989: 5). Because of its potential unfalsifiability, some linguists (nomina sunt odiosa) even went as far as to say that the theory of UG is not scientific.
Linguists will never agree on whether there is UG or not (for different reasons), but the generative paradigm is not pseudoscience. Even though I am no longer actively involved in generative research anymore nor do I find it particularly enthralling, as it used to be in the good olde days, I still believe Noam Chomsky is a great man and the most formidable linguist of our time.
If you need to understand the basics of Universal Grammar and if you don't have any formal training in linguistics, I would recommend something written for the general audience, perhaps "The Infinite Gift" by Charles Yang or "The Atoms Of Language: The Mind's Hidden Rules Of Grammar" by Mark Baker. Slightly more technical and better written (imho) is Chapter 4 "Universal Grammar" in "Foundations of Language" by Ray Jackendoff.
Then you might want to read Roberts 2017 (open access!) in The Oxford Handbook of Universal Grammar. I find this part very important and illuminating:
"Like any bold and interesting idea, UG has its critics. In recent years, some have been extremely vocal [...]. Again this is as it should be: all good ideas can and should be challenged. Even if these critics turn out to be correct (although I think it is fair to say that reports of the ‘death of UG’ are somewhat premature), at the very minimum the idea has served as an outstandingly useful heuristic for finding out more about language. But one is naturally led to wonder whether the idea can really be wholly wrong if it is able to yield so much, aside from any intrinsic explanatory value it may have."