In addition to other replies, this convention is not just for boolean values. It is for truthy or falsy values.
For example defined? method returns nil if expression is not defined or a string about expression if it is defined. The reason there is a ? at the end of this method is that you can use it in your if statements etc. to check an expression is defined or not.
defined? Object
=> "constant"
defined? undefined_variable
=> nil