The great baseball catcher, coach, and philosopher Yogi Berra once said, "We made too many wrong mistakes." You can be a mistake catcher too, brave Excel user!
More precisely, you can be an error catcher. Let's say you wanted to divide the numbers in one column by the numbers in another column. But look what happens when you divide by zero! AN ERROR!
We can catch this sort of thing and make the cell return something else with a conditional statement (IF) and the ISERROR function.
In English, this formula basically says "If dividing N7 by O7 results in an error, just return zero; otherwise, go ahead and divide N7 by O7."