。
She has a thorough understanding of her work.
Literal
She [topic-は] her own work [object-を] completely is-understanding.
完全に ('completely, thoroughly') is the adverbial form of the na-adjective 完全 ('complete'), formed by attaching に. 理解する ('to understand, comprehend') is a Sino-Japanese verb of cognition; with ~ている, 理解している marks the resultative state of having come to understand — Japanese treats 'know/understand' as a state reached, not an inherent property. Pairs naturally with 完全に for the idea of having fully mastered a subject.