user.routing.yml -- What is the significance of "_entity_form: user_role.default"?
In short, it means that route shows an entity form to edit the user role entity; in particular, it shows its default form.

In details, user_role is the ID for the Role class, which implements the user role entity.

@ConfigEntityType(
  id = "user_role",
  label = @Translation("Role"),
  handlers = {
    "storage" = "Drupal\user\RoleStorage",
    "access" = "Drupal\user\RoleAccessControlHandler",
    "list_builder" = "Drupal\user\RoleListBuilder",
    "form" = {
      "default" = "Drupal\user\RoleForm",
      "delete" = "Drupal\Core\Entity\EntityDeleteForm"
    }
  },
  admin_permission = "administer permissions",
  config_prefix = "role",
  static_cache = TRUE,
  entity_keys = {
    "id" = "id",
    "weight" = "weight",
    "label" = "label"
  },
  links = {
    "delete-form" = "/admin/people/roles/manage/{user_role}/delete",
    "edit-form" = "/admin/people/roles/manage/{user_role}",
    "edit-permissions-form" = "/admin/people/permissions/{user_role}",
    "collection" = "/admin/people/roles",
  },
  config_export = {
    "id",
    "label",
    "weight",
    "is_admin",
    "permissions",
  }
)


user_role.default is referring to the default form the annotation for that class describes.

"form" = {
  "default" = "Drupal\user\RoleForm",
  "delete" = "Drupal\Core\Entity\EntityDeleteForm"
}


In this case, it's the form implemented from the RoleForm class.

When you see _entity_form in a route, it always mean the route is showing a form implemented from an entity.