It's the number of hit dice the receiving creature has. For each hit die it gets a single hit point more.

An Orc having one hit die would get 1d6+4+1, while a human fighter level 6 would get 1d6+4+6.

Yes.

Thats the mundane way of recovering wounds from battle. Also called Natural Healing. Any character can recover wounds by resting for 8 hours, limited to once per day, regardless of how much he rests or how much aid he recieves.

The Heal skill simply allows a wounded character to recover faster.

You will notice that Treat Deadly Wounds says the character must recieve treatment no more than once per day, and no longer than 24 hours after being wounded. Which specifically prevents that type of treatment from being used more than once.

But for Long-term Care there is no restriction, other than spend 8 hours treating the patient(s). That is actually the slowest form of healing available.

Also keep in mind that the character recovering for 8 hours will recover only his double healing rate (2 hp per level) from your Long-Term Care, and not the quadruple (4 hp per level), as he requires a full day of complete rest.

For a complete rest, the character must make no stressing activity for the day if you want to recover. What is a stressing activity is left to the GM, but most assume that any skill check, combat, travel or spellcasting counts as stressing.

You can even Provide Long-Term Care with your downtime. Or while Travelling (which is a common use of the skill), as that is considered light activity for the healer.

Giving long-term care counts as light activity for the healer.

Which means you could even treat someone while recieving long-term care yourself. You simply cannot treat yourself this way, or recieve quadruple the healing as that requires complete rest.

A Mobile Hospital can be used to double the amount healed aswell.