To back up = make a "protection" copy to use if there are problems with the original.
"When my computer crashed, I lost many of my files. It's a good thing I backed them up."
A back up plan is like a plan b. It's a plan that you have just in case the first option isn't successful.
Example: Although Kierra's lifelong dream is to be a singer, she still plans on earning a degree in nutrition, just to have a plan b.
Create a sentence below using back up.