permissible to remain silent when seeing an evil action?

“Whoever among you sees an evil action, then let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart – and that is the weakest of faith.” Narrated by Muslim in his Saheeh. what does ''if he cannot'' mean? does it mean, iff he is **Physically** incapble of changing the evil or just **perosnally** hesitant to speak up ...?.. can someone please give me an example.. and explain what the prophet sallallahu alaihi wa sallam menat by ''if he cannot'' . **also Question 2** : according to this hadith , is it permissible to stay silent and only hate it with his heart?