This is the 99th most frequent Hungarian word.
teljesen
Completely; entirely.
In this sentence, 'teljesen' (completely) modifies the adjective 'csendes' (quiet) to emphasize the extent of the quietness.
Az este teljesen csendes volt.
The evening was completely quiet.
Here, 'teljesen' (completely) modifies the verb 'megértem' (I understand) to indicate the extent of the understanding.
Teljesen megértem, amit mondasz.
I completely understand what you are saying.
In this example, 'teljesen' (completely) modifies the adjective 'új' (new) to underline the absolute newness of the book.