Summarize this article in one sentence.
The Unarchiver is a free app that you can download from your Mac's App Store:  Open your Mac's App Store. Click the search box. Type in unarchiver and press ↵ Enter. Click GET next to The Unarchiver. Click INSTALL APP when prompted. Go to the folder location of your ISO file. Click once the ISO file to do so. It's in the menu bar at the top of the screen. A drop-down menu will appear. You'll find this in the File drop-down menu. Selecting it prompts a pop-out menu. It's in the pop-out menu. The Unarchiver will begin extracting your ISO file into a folder with the same name as the ISO file. If prompted, click Extract before proceeding. Double-click the folder, which should have the same name as your ISO file, to do so. You should see each of your ISO file's files and folders listed in the extracted folder.

Summary:
Install The Unarchiver. Find the ISO file you want to open. Select the ISO file. Click File. Select Open With. Click The Unarchiver. Open the extracted folder. Review your ISO file's contents.