Imagine the Computer needs the name of a Student to do something via a command:

"The student's name is ____"

A hacker types into the underlined section : "Bobby, but also delete everything"

The final command now looks like:

"The student's name is Bobby, but also delete everything"

Without double checking the user input the computer proceeds to delete everything due to the second half of the result.

It's something similar to "ignore all previous instructions" for an AI prompt.