ISO is an archive. Is a virtual image of a CD, DVD or Blu-ray. Even you don't have the CD or a CD-ROM, you can open ISO file as a CD. In other words, instead using a physical CD, you open an ISO file like would be a CD.
Files with ISO extension are like computer CD, just they are in another shape - a static file on your hard disk.
To open an ISO file you need to use a special software for that, the best on the market is Daemon Tools. You can find it on this page. Download it and install it on your computer.
Right click on Daemon Tools icon in the right and bottom side of your screen. Choose Virtual Devices (to open the ISO file like a CD-ROM, just it is in a virtual mode), go to Device 0 No Media and select Mount Image.