it
The German word 'es' is a pronoun that is used to replace a noun or a noun phrase in a sentence. It is used to refer to things, animals, or people, and it is often used when the gender of the noun is unknown or irrelevant. The word 'es' is also used as the subject of impersonal verbs, such as 'es regnet' (it is raining).
It is raining.
This statement describes the weather condition, where 'Es' functions as an impersonal pronoun equivalent to the English 'It' when referring to the weather or time.
It is a book.
'Es' in this sentence functions as a pronoun, referring back to a noun mentioned or understood in the context. The sentence could be used when someone is asking what object you are holding or talking about.
I am fine.
In German, 'Es geht mir gut' literally translates to 'It goes well for me.' Here, 'Es' doesn't refer to a specific thing; it's just a required functional part of the sentence.