This is the 10890th most frequent German word.
sandra
"Sandra" is not a German word but a proper noun, typically a female given name in German and other languages.
Here, 'Sandra' is again used as a proper noun, referring to the same name. In this context, 'Sandra' is the object of communication and is used in its dative case.
Ich habe heute mit Sandra gesprochen.
I spoke with Sandra today.
In this sentence, 'Sandra' is used as a possessive form, 'Sandras', indicating that the book belongs to Sandra. In German, proper nouns' possession is shown by adding 's' at the end of the name.
Das ist Sandras Buch.
This is Sandra's book.
This usage of 'Sandra' is as a proper noun, specifically a person's name, which is common in German as well as in many other languages. It serves as the subject of the sentence.