"This is a dream."
Translation:Ez egy álom.
The difference is hard to explain and also quite insignificant here. I personally think it's like... without any articles, you don't create a reference to a standalone substance, more like you create something like a weird adjective. Pushing this narrative to the current example, it's like "this covers the definition of dreams" without an article and "you can imagine a dream, right? now, this is one"
It's a more useful approach with occupations for example. "Egy orvos vagyok" sounds a bit meh for example. I'm not a newly introduced instance, you don't have to "imagine" a generic doctor just to assign it to myself, I'd rather say "I cover the definition of doctors". "Egy orvos leszek" is even worse imo, it starts to sound like "I will be just one of them, nothing special..." rather than personal development.