herunter
down
Expresses movement from a higher to a lower position; downward.
Er kletterte vorsichtig die Leiter herunter.
He carefully climbed down the ladder.

down
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down
Expresses movement from a higher to a lower position; downward.
Er kletterte vorsichtig die Leiter herunter.
He carefully climbed down the ladder.

'Herunter' (down) consists of 'her' and 'unter'. The direction 'her' describes movement toward the speaker. So 'herunter' means 'down, toward me'.
Ich bin oben, wirf den Ball zu mir herunter!
I'm up here; throw the ball down to me!
'Herunter' (down) is a common prefix for separable verbs such as 'herunterladen' (to download), 'herunterfallen' (to fall down), or 'herunterkommen' (to come down). In a main clause, the prefix stands at the end.
Pass auf, dass die Tasse nicht vom Tisch herunterfällt.
Be careful that the cup doesn't fall down from the table.
The expression 'herunter sein' (to be worn out) or 'mit den Nerven herunter sein' (to be at the end of one's rope) is a fixed idiom. It means that someone is completely emotionally or physically exhausted.
Nach dem Marathon war ich körperlich total herunter.
After the marathon, I was physically completely worn out.
In colloquial speech, 'herunter' (downloaded) is often used without the verb 'laden' (to load/download) to express that a download is complete. The question 'Ist es schon herunter?' (Is it downloaded already?) is very common.
Das Spiel ist riesig, es wird Stunden dauern, bis es herunter ist.
The game is huge; it will take hours until it's downloaded.
above / up
'oben' and 'herunter' both have to do with height, but they do not describe the same thing. 'oben' describes a position, or with 'nach oben' a direction toward a higher place. 'herunter' describes movement from up to down, or a decrease. Learners may confuse them because both appear in situations involving stairs, mountains, shelves, or values. Remember: 'oben' says where something is; 'herunter' says that something is moving downward or becoming lower.

Nach dem Sturm fallen kleine Äste vom Baum herunter.
After the storm, small branches fall down from the tree.
Mein Kollege sucht die Akte, aber sie liegt oben auf dem Regal.
My colleague is looking for the file, but it is at the top of the shelf.