What is the meaning of the safe_attribute value ?

Hi,

In all the core views of redmine, I see if statements that check this : @issue.safe_attribute? 'string'

My question is : what is the meaning of the existence of this variable ?

What is it ?
What is it's content ?
Can we safely use it at plugin level or modifying this value can have second effects ?

Thank you !

Best regards,

Redmine