。
She explained it over and over.
Literal
She [topic-は] that [object-を] repeatedly explained.
繰り返し is the noun/adverbial form of 繰り返す ('to repeat'); used adverbially before another verb, it gives 'repeatedly, over and over.' Pairs naturally with verbs of speech and explanation: 繰り返し説明する ('explain repeatedly'), 繰り返し言う ('say over and over'). The reduplicated stem 繰り (from 繰る 'to wind, to reel') captures the image of action winding back to its start — a fitting metaphor for repeated effort. 説明する is the standard verb for 'explain' in any register.