A disk image is a compressed file that mounts like a hard drive. It would be somewhat similar to viewing the contents of a zip file without extracting the contents.
When you open a disk image, it looks and acts like a hard drive. Apps are distributed on disk images. On some, you copy the app into Applications. Others might have an installer package that you run to install the app.
There should be "buttons" on each reply that allow you to mark a post as helpful or solved. You can only mark a reply with one of those, and you can only mark two posts as helpful and only one as solved.