An unless evaluates the then branch if its condition is falsey, and evaluates the else branch, if there’s any, otherwise. That is, it behaves in the opposite way of an if:
unless some_condition
then_expression
else
else_expression
end
# The above is the same as:
if some_condition
else_expression
else
then_expression
end
# Can also be written as a suffix
close_door unless door_closed?