Temporary hitpoints are added to your normal hitpoints. if you take damage, the damage comes from your temporary hitpoints first, until you run out of temporary, then the damage is applies to your normal hitpoints.
So if you have 100 hitpoints and 20 temporary, and take 6 damage, you'll be at 100 regular and 14 temporary. If you then take 15 damage, you'll be down to 99 regular hitpoints, and no temporary hitpoints left.
Once temporary hitpoints are gone, you can't heal them back.
If you gained 20 temporary hitpoints while you were at 37 out of 100 hitpoints, you'd then be at 57 out of 120 hitpoints. So, no, they don't actually heal anything.
-The Souljourner