NULL is a special value used in database products to signify that a cell or field contains nothing, not even the empty string or 0. It can also mean uninitialised, meaning that this cell has never been used before.
true and false are names for values that are actually numerical. true is equal to -1, and false is equal to 0. In QlikView you can use them interchangeably: it doesn't matter which one you use.
IsNull() is used - amongst others - to check whether a field or a table cell actually contains a value that should get your attention. You may prefer to act on IsNull() = true to fill some field with a default value (e.g. 'UNKNOWN') when it contains nothing, or to throw away a row when the most important field is missing.
It would help if you could tell us what exactly isn't clear to you instead of the Good/Not Good qualification.