Communities are similar to projects and workspaces. They organize volunteer opportunities in various locations and campaigns. You can create a community based on causes, office locations, or any other type you like to manage user groups for opportunities. Every opportunity hosted is associated with a community. Communities can only be created and viewed by admins though. A user with no admin rights cannot create a new community.
Below is a video walkthrough to create a new community. It's a simple 2 step process to create a new one.