So, down at the top of your screen, above the view of a thread, is a oval button that says "Resolved" (at least, it's there on desktop view). It allows the author to say "My question is answered." Honestly, very view people ever use this button. We staff also have a button that we can use to give a thread a second resolved dot. This only ever gets used in our technical threads about things like "Somethings wrong with our forum, we've got this glitch." And then, once the glitch is fixed, we mark it resolved. But, um, we usually forget to ever do that. The button really never gets pushed.
Wiki threads are threads that can be edited by either anyone or just staff an the original poster. We usually use these for things like the
PEP badge pages, or pages that are to be edited like this one
https://permies.com/wiki/76253/Edible-Plants-Shady-Wet-Areas.
Anyone can make a wiki page (the option is down by the "attachments" tab when you're making a new post), but unless you want it to be edited by others to further add more knowledge to it, you probably don't want to make it a wiki (you'd be amazed at the amount to singles ad posts that the person make wikis!)