A UPS driver will make 3 attempts to deliver your package to your door and if you don't sign for the package on the third attempt then the package will be sent back to the sender as non deliverable.
The UPS driver will leave a hanger on your door where you can sign the hanger as a signature allowing the UPS driver to leave the package in a safe location if you're not there.
But if you don't sign the door hanger or are not at home to sign for the package then after the 3rd attempt to deliver the package UPS will send the package back to the sender.
If the package does not require a signature then UPS will leave the package in a safe location by your door or somewhere else.