This is the 333rd most frequent Japanese word.
完全
Complete; perfect.
Here, '完全に' (completely) is used as an adverb to describe the degree to which the theory is understood.
この理論は完全に理解されているわけではありません。
This theory is not completely understood.
'完全な' (perfect) functions as an adjective modifying '結果' (results) to indicate a state of flawlessness or completion.
彼は完全な結果を目指して努力している。
He is striving towards achieving perfect results.
In this sentence, '完全に' is employed as an adverb emphasizing the thoroughness of the action '確保する' (to ensure).