Translation:You will arrive by eight, please.
I don't think this sentence makes sense in English, and probably Welsh too. Changing it to a question, as in, "Gyrhaeddwch chi erbyn wyth, os gwelwch chi'n dda?" would make more sense.
By including the chi after the imperative , it is just a way of emphasising that it is YOU whom I am telling to arrive by eight:
- Cyrhaeddwch erbyn wyth! - Arrive by eight!
- Cyrhaeddwch chi erbyn wyth! - You - arrive by eight! You will arrive by eight!