Because computers operate using strict logic, invalid input may produce unrecognizable output, or "garbage." For example, if a program asks for an integer and you enter a string, you may get an unexpected result. Similarly, if you try to open a binary file in a text editor, it may display unreadable content.
GIGO is a universal computer science concept, but it only applies to programs that process invalid data. Good programming practice dictates that functions should check for valid input before processing it. A well-written program will avoid producing garbage by not accepting it in the first place. Requiring valid input also helps programs avoid errors that can cause crashes and other erratic behavior.
NOTE: Because the related terms FIFO and LIFO are pronounced with a long "i," GIGO is typically pronounced "guy-go" (not gih-go). This also helps avoid confusion with the prefix "giga," which is pronounced with a soft "i."