Translation:I turned off the light.
No. It's an adverbial particle in this usage and part of the phrasal verb "to turn off". Many words have not just multiple senses, but multiple parts of speech. You can tell the difference if the sentence still retains part of its meaning by removing the suspected prepositional phrase:
- He fell off the cliff -> He fell
- He turned off the light -> He turned?
Read the explanation of the use of 把 here! Basically, it's a way to change the structure of a Chinese sentence. https://resources.allsetlearning.com/chinese/grammar/Using_%22ba%22_sentences
The way I learned it was similar to "I take something and do something with it"
我把灯关了 (I take the light and turn it off) I turn off the light/I turn the light off
我把我的想法写下来 (I take my thoughts and write them down) I write down my thoughts
你可以把那个肉给我吗？ (Can you take that meat and give it to me?) Could you pass the meat?