A construction worker stands in front of a red truck which has the word 'hot' on the side.
A worker standing in front of his truck which has writing on it.
A construction worker is a type of worker. A red truck is a type of truck. Having the word hot on the side means the same as having writing on it.
A construction worker is a worker standing in front of (red) truck which has writing (hot) on it.
On it is a simplified version of on the side, and for a word to be on something it has to be written down aka it has to be writing.