。
The airplane took off one hour late.
Literal
Airplane [topic-は] 1-hour late took-off.
遅れて is the te-form of 遅れる ('to be late'), used adverbially to describe the manner of takeoff: 'took off in a state of being one hour late.' This te-form-as-adverbial pattern is one of the most flexible structures in Japanese for layering manner onto an action.