If when the courier arrives there is nobody home, they will leave the item in a safe place and leave a calling card stating where your item was left.
If they do not feel there is a safe place, they will either leave with a neighbour or they will take the parcel back to the depot. A calling card should be left either way.
If your parcel requires a signature and you are not home to accept delivery, the courier will re-attempt the next working day or may contact you to make alternative delivery arrangements.