This is the 1026th most frequent Russian word.
защитить
To protect or defend.
Here, "защитить" means "to protect" or "to defend", commonly used in contexts involving protection from harm or danger.
Я хочу защитить свою семью.
I want to protect my family.
In this context, "защитить" refers to "defending" an academic thesis, a standard term in Russian academia.
Он должен защитить свою дипломную работу завтра.
He has to defend his thesis tomorrow.
The word "защитить" here carries the meaning of providing defense or advocacy, specifically in a social or moral sense.