I find that sometimes the technical description of the Excel functions can be a little verbose and complicated. However, I think if you look at the examples provided, it makes a lot more sense. Take a look at the examples Excel provides in its help on "IF worksheet function".
In a nutshell, the function works like this:
=IF(condition to check, value to return if condition is true, value to return if condition is false)
Note: There are 3 parts to the formula, separated by commas.