This is the 522nd most frequent Russian word.
защиты
"Protection" or "defense."
The word 'защиты' here is used in the genitive case to indicate possession or the mechanism related to protection.
Эта дверь имеет специальный механизм защиты.
This door has a special protection mechanism.
In this sentence, 'защиты' appears in the genitive case, referring to the event of defending the thesis.
Во время защиты диплома студент волновался.
During the thesis defense, the student was nervous.
Here, 'защиты' is used as a part of a phrase 'защиты окружающей среды,' denoting the genitive case relation to environmental protection.