The default filter returns the passed default value if the value is undefined or empty, otherwise the value of the variable:
{{ var|default('var is not defined') }}
{{ var.foo|default('foo item on var is not defined') }}
{{ var['foo']|default('foo item on var is not defined') }}
{{ ''|default('passed var is empty') }}
When using the default filter on an expression that uses variables in some method calls, be sure to use the default filter whenever a variable can be undefined:
{{ var.method(foo|default('foo'))|default('foo') }}