What does Rules token "node:unchanged" mean?
Terminology

"unchanged refers to the value before something (eg, a field of entity user or node) gets updated (the old value that it used to be).

"changed refers to the value after something (eg, a field of entity user or node) gets updated (the new value that it will become).

Examples

Have a look at my answers to these question for some typical usecases of it:


Monitoring field changes in custom content type
How to implement admin verification for selected user profile fields?
How to include only an updated custom field in an email created using Rules?
How to use Rules to email users when any field(s) is changed in a node (and include the unchanged/changed values for that field)?