NOT is just opposite of YES, or if anything is true then not makes it false.
- NOT - if the Condition without NOT is TRUE it's FALSE
- =NOT(unitsInStock > 100) -> if the units in stock is NOT greater than 100 it's true
- =NOT((unitsInStock + 100)>500)
Create following columns for understanding NOT:
lessThan100 (Calculated Column)

=(NOT([UnitsInStock] > 100)
Output: (Modified View)

another example let's create the following column for this example:
getTo500 (Calculated Column)

=NOT(([UnitsInStock] + 100) > 500)
Output:
