When someone copies a song on to her computer's hard drive from a portable musical source (like a CD), this is called "ripping" a song. You can also rip music from a flash drive or an MP3 player.
In most cases, you simply need to pop the CD into your computer and follow the prompts you are given. A box will appear asking if you want to rip the song, click "Yes" to start ripping.
The word comes from a time in the print media industry's history when images had to be copied directly from the original using a process called Raster Image Processor. This is where the term "rip" comes from.