What is the use of ISUNCHANGED() in validation rules in Salesforce?

I guess you meant ISCHANGED()

ISCHANGED() – Compares the value of a field to the previous value and returns TRUE if the values are different. If the values are the same, this function returns FALSE.

You can have the example here:

https://developer.salesforce.com/forums/?id=906F00000008wIVIAY