While looking at badges, I saw something called edit wiki tag. Although I searched the site a bit, I couldn't find anything like a wiki. Actually, I don't quite understand what a wiki is.
My question is what does this wiki mean?
Tag wikis are the descriptions of the tags on this website. To visit a tag's wiki page, click on the tag to visit its question list, then click the "Learn more..." link to follow through to the full wiki page.
Tag wiki edits are crowdsourced like much of the other content on this website. For more details and for editing requirements and guidelines, please read this FAQ entry on Meta Stack Overflow:
If you're hunting for the badge, I might recommend paging through the popular tag list until you find a tag that could use some description but doesn't yet have an excerpt. Here are a few examples (feel free to edit):
It's also helpful to browse this popular tags list because it gives you some examples of what good tag wiki excerpts should look like.
Finally, if you don't feel like writing a longer wiki article, it is perfectly acceptable to submit only an excerpt.