What does CD -rom stands for?

Compact Disk Read-Only-Memory.

Read-only-memory refers to a long-term memory solution. You have it in your computer under the form of a hard drive. Files and programs are stored in Read-Only-Memory then transferred to Random-Access-Memory (RAM, usually called "memory" by the average person) to work with.

Compact disk is just the name of the technology used to store the data.