If UPS tries to deliver a package to your door but you're not there to sign for the package then UPS will leave a note on your door letting you know that they attempted delivery that they needed a signature for and they will return the next day with the package and try again.
You can sign the note on your door which is good enough as your signature to let the UPS driver leave the package at your door or in a safe location.
The UPS driver will make at least 3 delivery attempts to deliver the package to your door and then will return it to the sender after 3 delivery attempts.
So either make sure you're gonna be home to sign for the package or sign the note on the door so the driver can leave the package.