What does the question mark at the end of a method name mean in Ruby?

Question

What is the purpose of the question mark operator in Ruby?

Sometimes it appears like this:

assert !product.valid?

sometimes it's in an if construct.

Answer 1

It is a code style convention; it indicates that a method returns a boolean value (true or false) or an object to indicate a true value (or “truthy” value).

The question mark is a valid character at the end of a method name.

https://docs.ruby-lang.org/en/2.0.0/syntax/methods_rdoc.html#label-Method+Names