What unit does the radiation indicator show?

The rate is 1% of HP per 10 rads, meaning you lose 1% of your max health per 10 radiation you take. This is indicated by the red bar that grows from the right side of your health bar when you incur radiation. If your 100 health max character takes 200 radiation, your max health is now 80 until you lower your radiation.

Using this formula, and confirmed by the max value on the "radiation" indicator on the right side of the Pip-Boy, 1000 radiation is lethal. 1000 radiation would reduce your max hp by 100%.

When you are receiving radiation, you'll get the indicator by your health bar. +2 rads would indicate you are taking 2 radiation per second. The time it would take to kill you while receiving x rads per second would be: (1000 - CurrentRadiationLevel)/x seconds.

The lethal radiation dose is same as in previous games 1000 rads. So you will be dead after 500 seconds of exposure too +2 rads a second.