What is meant by 'fixing errors'?

It means just what it says: something was wrong and needs to be corrected. Often it is a piece of code that doesn't do exactly what was intended and sometimes it is data that was entered incorrectly. For an example, if a program included the statement "divide A by B giving C" it would crash if B were equal to zero. If zero is an acceptable value for B, then the code would be changed to perform the divide only when it contains a non-zero value. If zero is not a valid value for B, then the data must be corrected.