。
My mother is preparing breakfast.
Literal
Mother [topic-は] breakfast [possessive-の] preparation [object-を] is-doing-[polite-ます]
朝食の準備をしています = 'is doing breakfast preparation' = 'is preparing breakfast.' The polite ongoing ~ています highlights the action in progress. 朝食 (ちょうしょく) is slightly more formal than 朝ごはん — the latter is what you'd say at home, the former what you'd see on a hotel menu.