Most information about MSE can be found through the Help Center, which can be accessed from the navbar at the top of the site:
The Help Center has a section labeled "Privileges". Click on the only link in that section to find a list of privileges which are earned at various levels of XP. This list is clickable. Clicking on the link for 5,000 XP gives you a description of what the tag wiki editing powers entail.
From that link:
What are tag wikis?
Tag wikis are a general introduction to a topic defined by that tag. They're a place to consolidate and form a sub-community around a tag. They contain, among other things, frequently asked questions in the tag, top users in the tag, and best recent answers in the tag.
Voting on tag wiki edits
All registered users can submit edits for tag wikis. When approving these edits, ensure that:
- All the content is original or attributed properly
- Wiki excerpts are concise and contain an objective description of the tag
When you approve tag wiki edits the person submitting the edit will gain 2 reputation points.