In most programming languages, a Boolean is the value type to denote that something is either true or false, on or off, yes or no, 1 or 0. In the Hearts of Iron IV scripting language, a Boolean is noted as either "yes" or "no".
has_war = yes
The above Trigger for example, uses a boolean: the word "yes". Triggers in particularly check if specific conditions are met, to which the answer of the entire trigger end up being a yes or no answer themselves as well.
Unfortunately, the scripting language does not support more advanced boolean algebra than this. but do take a look at the AND, OR and NOT operators.