Vlookup is one of the functions I use the most. It can locate data within a table and return a value. One of the most confusing things about the function is whether or not to include the True or False at the end.
When using the Excel Function wizard, it argument in the function is called Range_lookup.
Simply stated, use a TRUE or 1 when you want an approximate match. Use a FALSE or 0 when you need an exact match.
Use TRUE when the data in the lookup table does not contain all of the possible data points. For example, in the figure below, if you want to look up the tax rate you would use an approximate match since all incomes are not in the table. If the net income were 100,001, it would return a rate of 39%.
It finds the closest value that is not larger than the lookup value.
In order to use an approximate match, your data must be sorted as well. Otherwise, use a FALSE or exact match.
For more examples, see this YouTube video: