Filters
default
value | default
Falls back when the value is nil, false or empty — and, unlike stock Liquid, also when it is an empty string after trimming.
Where this differs
A setting left blank in the editor arrives as an empty string, not nil. Stock Liquid would render nothing; this renders the fallback, which is what the author meant.
Example
{{ section.settings.heading | default: 'Untitled' }}