It's when you can't make your own music so you just copy and paste someone else's work and change it slightly.
Examples: half naked and almost famous - mgk. Rip-off of "young blood" by naked and famous.
"The show goes on" - (I think lupe fiasco). Rip off of "float on" by modest mouse
"Yolo"- lonely island. Rip-off of "whirring"- the joy formidable.