The community portal of a wiki is where users can organise information for editors (and to a lesser extent, users).
As most of the content of a wiki is in the articles, it can be difficult organising the editors to work on what needs to be worked on. For brand new wikis, you generally just want content, but very soon, you will need to ensure that the content reaches a good standard - which is where the community portal comes in.
You can use this page for a wide variety of things - for examples, listing tasks to be done, giving easy access to the various special pages around the wiki - or for really big projects, give links to individual guideline and task pages.
Some wikis also use the community portal's talk page as the main talk page of the wiki.
You can view Wikia's default community portal design on w:c:starter:Project:Community Portal.