A correction
The most useful thing you can send. If a sentence on this site is wrong, misleading, or out of date, point at it. You do not need to be gentle, and you do not need to be certain. We would rather check ten reports and find one real error than miss the one.
Substantive corrections are noted on the article itself, with the date, as set out in the editorial policy.
A topic suggestion
If you looked something up recently and every result was useless, that is exactly the kind of thing this site exists for. Tell us what you searched for and what you failed to find.
Republishing or citation
Quoting a paragraph with a link back never needs permission. Republishing a whole explainer does, and it is usually granted. Say where it would appear.
Something else
Everything that does not fit the above. It reaches the same inbox.