The little word es appears everywhere in German. At CEFR level A2 you meet its most common uses: as a personal pronoun for neuter nouns, with weather and time, in impersonal expressions like es gibt or es tut mir leid, and sometimes as a placeholder at the start of a sentence.
The little word es has four jobs in German: as a real pronoun for a das-noun, as the obligatory grammatical subject in weather/time expressions, inside fixed phrases, and as a placeholder at position 1 of a main clause.
Replaces a Neutrum-Nomen – es means it.
With weather, time and impersonal adjectives es is the grammatical subject – obligatorisch, never omitted (Regnet. is wrong).
High-frequency phrases that always contain es:
If nothing else fills position 1, es stands there as a dummy – and disappears as soon as another element moves to position 1.
Es gibt means there is / there are. It is always singular and takes the Akkusativ: Es gibt einen Park. / Es gibt viele Leute.
Weather verbs like regnen, schneien or adjectives like kalt, warm, sonnig have no real subject. German always needs a subject on position 2, so es fills the slot: Es regnet. / Es ist kalt.
Usually no – es is needed for weather, time and fixed expressions (es gibt, es tut mir leid). Only the placeholder-es at the very start disappears when another element moves to the front: Es kommen viele Gäste. → Heute kommen viele Gäste.
No. In Wie geht es dir? the es is an impersonal subject – it does not stand for a noun. Learn the expression as a fixed unit: Es geht mir gut. / Es geht mir schlecht.