A general explanation is good, showing examples is better.
Why isn't it the following?
A general explanation is good, showing examples are better.
How can this sentence be analysed?
Robertd has it right in that "showing" is acting as a singular noun.
Consider that in the sentence in question only the predicates are grammatically parallel ("is good" and "is better"). We certainly could, however, turn both subjects into gerund phrases:
Explaining something generally is good, showing examples is better.
Or, to strip it down:
Explaining is good, showing is better.
Or, we could go the other way and make the subjects both simple nominative phrases:
A general explanation is good, examples are better.
In this case (above), the first subject is still singular, but now the second subject is plural. We could, of course, also make both subjects plural to make the predicates mirror each other better, as in the original:
Explanations are good, examples are better.
Or, to add adjectives...
General explanations are good, specific examples are better.
Now, could we have plural gerunds? They are normally a bit awkward (and many would be entirely non-standard), but perfectly grammatical as in:
Tellings of stories are good, showings of emotion are better.
Here, I have avoided "explainings" because that would be a completely non-standard usage, but it is possible to get away with "showings" and "tellings."
(Incidentally, why doesn't "explaining" work as a plural gerund? It probably has to do with the imperfective nature of the verbal form of "explain" which does not lend itself as easily to completed instances of explaining, as does showing or telling.)