This is not an expression but a keyword, allowing template designers to keep the content of a tag as an alternative value if an error occurs, or if something is not defined.

<span tal:define="myVar path/to/possible/var | default">
  default my var value

<span tal:content="some/var | other/path | default">
  no some/var and no other/path found here

<a href="unknown.xhtml" title="Unknown page"
   tal:attributes="href item/href | default; title item/title | default"
   tal:content="item/title | default">Unknown page</a>

Above examples introduce the '|' character that allows the definition of alternatives for defines or prints.