This is the 6478th most frequent German word.
eins
"One" (the number).
The word 'eins' is used as the number one to convey its property as a prime number.
Eins ist eine Primzahl.
One is a prime number.
The word 'eins' is used as a pronoun to represent an unspecified item, in this case an apple.
Ich nehme eins von den Äpfeln.
I take one of the apples.
The word 'eins' is used as a substitute for a previously mentioned item, serving as a pronoun.