USPS: what's it mean when a package says "out for delivery" but it's not delivered?

It means that the letter carrier or parcel truck driver has taken it out on the delivery route and will deliver the package when he or she gets to the part of the route that has your house on it. They may have a truck full of parcels. They can't deliver them all at the same time. The one who delivers your letters may not be the same one who delivers parcels since a person on foot cannot carry all the parcels going to all the houses they deliver letters to.

Sometimes packages don't come with the regular mail delivery, but on a separate truck.

It means it was suppose to be put on the truck for delivery, it may not have got there, the driver may have forgotten to deliver it, or it may be on a different truck than the normal carrier, or the carrier did or may have tried to deliver at the wrong house.
Last time this happen to me I went to the post office the next day, the notice attached to the package (incorrectly) said nobody was home to accept delivery, and that notice was left on my door (it wasn't). When I was leaving I recognized a woman in the parking lot (it's a small town), turns out she is dating a carrier, she said likely the driver forgot, but I would get no serious response from the local post master, he is a real ***.

"The nine most terrifying words in the English language are: I'm from the government and I'm here to help."

- President Reagan Aug. 12, 1986

When it says out for delivery, it means it is on the truck and on the way to your home. If you package is a very small item. There is a chance they will put it in your mailbox instead of place it in front of your door. The time range you will receive it after it said out for delivery is anytime from morning to 8pm. If you still dont see it, then you should check with them, call them or whatever.