I downloaded a .toast file, but I don’t know how to open it. Do I need a software to open them?
It's an image file created with Roxio Toast (www.roxio.com/enu/products/toast/)
Toast Images are usually ISO compliant
You should be able to open it with built-in Disk Utility or change the file extension from "toast" to "iso" and open it with your preferred burning program.