This is the 21st most frequent Greek word.
αυτό
"Αυτό" means "this" or "it" in English.
In this sentence, 'αυτό' is a demonstrative pronoun used to refer to an unspecified object, meaning 'this'.
Αυτό είναι ένα βιβλίο.
This is a book.
Here, 'αυτό' is used as a demonstrative pronoun to introduce a relative clause, meaning 'that' or 'what'.
Θέλω να καταλάβω αυτό που λες.
I want to understand what you are saying.
In this usage, 'αυτό' acts as a demonstrative adjective, modifying the noun 'το βράδυ' (the evening) to mean 'this' evening.