The Compact Disc Interactive (CD-i), is an interactive multimedia CD player and format developed and marketed by Philips. in 1991/1992. Notably, it featured intellectual properties from Nintendo, such as Mario and Zelda, due to previously having tried to develop a CD add-on for the SNES. Their takes on Nintendo intellectual property were so infamously terrible that you would be hard-pressed to find anyone who doesn't think of them when you mention the CD-i. However, there were a few cult classic games on the system.
Cdiemu is payware that hasn't been updated since 2006. Its compatibility isn't very good and it only responds to mouse movements for control, making most games unplayable unless you use an additional program called Mah Boi.
MESS has support for CD-i, which is also incomplete, but much easier to control, and it actually emulates better audio than Cdiemu in some cases. It is recommended to start the emulation with the game already loaded, either with the game picker or through a soft reset after mounting the disk.