What this concept is saying boils down to the following:
It is often simpler for us to understand questions when they are phrased in terms of "true" rather than "false". So when a question occurs that asks you to find the answer that is "not necessarily false", you can convert it to "could be true" in your mind for ease of use.
Knowing the equivalent statements of true and false (e.g. Must be true and Cannot be false) is very helpful because it saves you from confusion in the less common instance that the question stem phrases its directive in terms of "false".
If you feel comfortable understanding questions in terms of "false", feel free. But be aware that there is this method to help with potential confusion!