Special door in the Vatican: Only Open Once/Century?

Factual Questions
1

I read once about a special door, in the Varican City. It is only opened when the Pope proclaims a “jubilee” year-this is usually once every century.
What is the sgnificance of this door? Is it used to transport aything?

2

Cecil has done this. Here’s the column on it…

3

Wiki entry when you type in “Jubilee” and look at “theology”: Jubilee in the Catholic Church - Wikipedia

Google result from “vatican holy doors jubilee”:
http://www.vatican.va/news_services/liturgy/documents/ns_lit_doc_20010106_chiusura_en.html

4

Holy doors.

There’s four of them.