Temporary Hitpoints
When granted temporary hitpoints, you gain a pool of extra hitpoints. These extra hitpoints are separate from your Hitpoint total, and should be tracked separately. Any damage suffered is subtracted from the temporary hitpoints first, before reducing your actual hitpoints. After the temporary hitpoints expire, any remaining temporary hitpoints are lost. You can gain temporary hitpoints regardless of your current hitpoint value, even while at maximum hitpoints.
Temporary hitpoints do not stack, so if you receive additional temporary hitpoints while you have temporary hitpoints remaining, take the better of the two values (the new value or your current value).
Temporary Hitpoints do not last long and expire whenever you complete a rest (unless stated otherwise).