The meaning of Urbi  et Orbi:

An Urbi et Orbi blessing is  usually given at Christmas and Easter. It means, “to the city and the world,” and is given for the benefit of all humanity.

The meaning of Plenary  Indulgence:

Simply stated, plenary indulgence is forgiveness without punishment attached; in other words,  a fresh start. 

For a more detailed description, click here.