There are different names for it:dummy,empty,formal and, perhaps, more (We are not talking about a personal pronoun,a substitution for an object, an animal or an abstract noun). They are subdivided into impersonal it (no antecedent,the subject can't be identified )(1) and anticipatory it or introductory it(2) with the real subject coming later in the sentence.
1.It is sunny.
It is hot in the room
(illustrate a dummy/empty impersonal it.)
2.It is difficult to learn English.
But the second sentence has a meaningful it because it refers to the fact that to learn English is difficult. It here =to learn English.
It can refer to a whole fact,event or situation.
It's difficult to learn English. (To learn English is difficult).
It's awful --everybody is ill,and our central heating isn't working.(The situation is awful).
It in these sentences is dummy/empty, introductory/anticipatory.
Not all dummy it-subjects are impersonal, but any impersonal it-subject is dummy/empty.