"Ask Peter" refers to the Peter Answers virtual tarot website. It is a practical joke site where users are able to prank their friends into believing that "Peter" knows all.
On the website, users are instructed to write a short petition to ask "Peter" to answer the question, then write the question in a different field. The trick is to enter a period, followed by the answer, followed by another period in the petition field. The way the website is coded, the answer shows up on the screen as text asking Peter to answer the question. This is helpful to make sure the subject of the practical joke is unaware that the user gave the answer. When the user submits, the website gives the answer the user wrote in the petition box, leaving the subject of the practical joke surprised and confused as to how "Peter" is able to know the answer.