Entity nodes

The following text I found in the DOM specifications I have not been
able to understand:

"The DOM Level 2 does not support editing Entity nodes; if a user wants
to make changes to the contents of an Entity, every related
EntityReference node has to be replaced in the structure model by a
clone of the Entity's contents, and then the desired changes must be
made to each of those clones instead. All the descendants of an Entity
node are readonly."

Can u please make it clear,
Thanks and regards,
Hari.