This is used to display an edit form for an entity. The value should be the name of the entity:

_entity_form: node.default
There are also some optional parameters that may be added in the defaults section of the route.
- _title: The page title of the route
- _title_context: Any additional information for the title sent to the t() function
- _title_page_callback: A PHP callable, typically in the format class::function, which returns the page title for the route