What does roll back mean in case of Stack Exchange sites? Does it mean to just delete the posted questions and answers or something else?
It reverts the post to the previous revision overriding an edit revision. After rolling back, the revision to which you have rolled back will be shon in the revision history.
From help center:
A rollback reverts a question or answer to a previous version in the edit history. The rollback action itself then appears as the most recent item in the edit history.
It is explained in detail in What is a 'rollback'? on Meta Stack Exchange.
When we rollback an edit for the first time, we will be awarded a badge named "Clean up".
Remember: A roll back should be done only to override a superfluous or a non constructive edit. See Why can people edit my posts? How does editing work?