What is too obvious to explain in a scientific paper?

Symbolic notation must always be defined in a paper. The reason is that there are not that many symbols and lots of things they can be used to represent. Thus, even if every class you ever took used v to represent voltage, other people are using it to represent velocity, vertices, values, etc. See, for example, this far-from-complete Wikipedia list of how Greek letters are used.

At a minimum, each symbol should be defined the first time that it is used. It is also helpful to your readers to provide a table of important symbols and to refresh their memory on definitions of critical symbols from time to time. This can often be done quite simply and efficiently within the sentence where the symbol is used, e.g.:

we consider a particle traveling at velocity v through a magnetic field of strength B(x), where x is the position of the particle in space.

A rule of thumb is: The article (including its references) should be self contained. Of course "self contained" is pretty hard to pin down exactly but as an example: If you use some β, then either define it or (at minimal) write something like "we follow the notation of X" with some standard book X. Also note that this usually does not cost you too much space because either something is very simple to define (e.g. writing the "c is the speed of light" is enough) or there is a good reference.

Another rule is that if the notion is uniquely defined in your field and taught to every undergraduate than you don't need to define it. From above: c can be something different from the speed of light, even within Physics, but a continuous function does always mean the same thing (once the topologies are fixed which should be from the context) as also "electric field" always means the same.

What is too obvious to explain in a scientific paper?

It all depends on your audience.

In The Structure of Scientific Revolutions, p. 20, Kuhn laments how scientists pick up where textbooks leave off and specialize their writing into "brief articles addressed only to professional colleagues," those "whose knowledge of a shared paradigm can be assumed, and who prove to be the only ones able to read the papers addressed to them."

In general, it is better to broaden one's audience by simple explanations and definitions that do not individually require significant expansion of the paper length (including by citation to longer explanations that would significantly lengthen things), than to limit oneself to a smaller audience whose actual knowledge matches your assumptions.

Here, you're getting clear feedback from the editor/reviewers about things that need to be clarified, so definitely do those things. The editor knows your audience better than you do, and the request is quite reasonable (even much more light than requests for changes often are).

Symbols should almost always be defined, and acronyms should be spelled out on first use, even if well known. This can be done within a sentence (e.g. ", where c is the speed of light in a vacuum") or parenthetical (e.g. "the New York Stock Exchange (NYSE)").

For other terms with specific meanings, it is helpful to define those specific meanings, to make it clear in your paper and help readers distinguish your specific meaning from all the other misuses of the term they've heard. Where specific definitions vary (and they do, a lot more often than you'd expect), it's helpful to cite a source for that definition just as you'd cite a source for any other fact you've included from the literature.