Stands for "Garbage In, Garbage Out." It means that if invalid data is entered in a computer program, the resulting output will also be invalid. So if a program asked you to enter a letter of the alphabet and you decided to be funny and enter "3.14159", there's a good chance the results you would get back would be pretty messed up, or "garbage." Because we computer users aren't always smart enough to enter valid data, programmers have to take extensive measures to prevent GIGO errors.
Updated: April 23, 2011