Electrical timer install question?

One step at a time usually the hot conductor is on the top of the switch. The other black is the switch leg up to your light . The splice of black conductor located towards the rear of the box is to carry the power to other parts of the circuit and they should be hot unless you have not turned the power off to this circuit do so now to avoid shock. The splice of white conductors is the neutral splice this is where you add the white off of the switch. I hope I explained the box easy enough for you to understand I'll watch for your question and try to answer it for you if you run into problems, good luck.