It's more like 'into' in meaning – it describes a direction "into" something. Jag stoppade in brevet i kuvertet 'I put the letter into the envelope'
this results in Brevet är i kuvertet 'The letter is in the envelope'
Ok but why IN and not I. This is the first sentence I have seen that specified the use of IN over I. I have seen IN I used together but never separate.