A Wiki is a knowledge base - like this one - where pages can be published to be read publicly, or in other cases limited to certain groups or even just for oneself. Wikis can be used for anything from documention for a project, a central collection of information for certain topics or just as a personal knowledgebase.
While it may seem like this Wiki is the obvious answer, there are many Wiki systems on libre.moe that can be used for certain things.
Wiki.js is the system used for wiki.libre.moe - this very wiki. Is is meant to document various aspects of this website and domain, and the services made available here, but also includes some other entries regarding free software. Users on libre.moe can join in and request to also be able to publish articles here.
GitLab is a system for managing Git-repositories, tracking issues and providing DevOps tools. It can be used for private and public projects and has a dedicated Wiki feature, where each project can also publish a Wiki alongside it. It's not as feature rich as other software, but works well and has it's use case. As a user on this site, you can use GitLab's Wiki-feature for your projects, even if the sole intention of a project is just it beeing a Wiki.
As opposed to Wiki.js, GitLab's Wiki is tied to the corresponding project and/or group, meaning as the owner of a project or group, you can administer permissions yourself and keep it under your control. You can create multiple Wikis, instead of using just one, centrally managed instance.
The retired service Gitea, which has moved to git.langrock.info, you had a compareable experience to GitLab's Wiki, hence the same can be applied to it.
Seafile, originally on this site, but now on seafile.langrock.info, allows authorized users to create personal Wiki, which are meant for private or limited group usage, resembling more of a Notion-style personal knowledgebase, as opposed to a Wiki like Wiki.js or GitLab, which are generally intended and designed for a more public target audience.