Conditional statements react based upon whether the top stack item is true or false.
Most of the time, you are comparing two stack items, therefore you need operators to handle comparisons. Here are a few of them. In all cases, non-zero is true and zero is false. The IF-THEN statement either does something or lets the program keep on moving. If the flag is zero, or false, the sun is unable to penetrate the cloud cover and I just read a book indoors.