This is the 627th most frequent German word.
ist
"Is" (third-person singular present of "to be").
Here, 'ist' is used as the third-person singular form of the verb 'sein', representing the state of being or location of the book.
Das Buch ist auf dem Tisch.
The book is on the table.
'Ist' here is indicating a profession in the third-person singular, used for the state of being in a professional sense.
Sie ist Lehrerin von Beruf.
She is a teacher by profession.
In this context, 'ist' connects the subject 'alles' with the predicate 'wahr', expressing a statement of truth.