Instant Death

In the event that something happens to your character that would be totally fatal, the DM will declare as such. For example, you fail a check to avoid a 50 foot pit fall trap into some spikes. The DM will not roll damage, and your character will die instantly. They will still be able to be resurrected as normal, providing the nature of their death allows it to happen. (egYou are able to recover the body in time to cast the spell).

Instant Death From Damage

If you take enough damage to reduce your health to 0 and then deal an amount of damage equal to your current amount of maximum hit points, you are dead. You bypass saving throws and the dying condition, and die.
The DM does not utilise this rule until your characters are at least level 3.