Try to make the post substantively better when you edit, not just change a single character. Tiny, trivial edits are discouraged.
So, basically, this covers most minor spelling and grammar fixes. These are not only discouraged, but actively blocked: in many cases, a spelling error cannot be corrected unless the person correcting it can find at least 5 other characters they can justify changing at the same time. Why is this done? If someone misspelled a word in an otherwise good question or answer, and another citizen corrects that misspelling, without changing anything else, then the latter person has left that question, and therefore this site as a whole, better than they found it. What is the upside to discouraging that?