"Amen" and "Hallelujah" are words used in the Christian religion. They come from the Hebrew language, and are commonly heard in the Christian church.

Amen means "Let it be" or "Truely." After someone says a prayer, you say "Amen."
Hallelujah means "Praise God" or "Let us praise God."
Peanut Butter is a popular American food eaten on bread. It is made from peanuts and oil. It looks like this http://www.slashfood.com/media/2006/01/peanut_butter.JPG

"Amen,halleluyah,peanut butter" is an irreverent (joking) mix of words. He is "Thanking god" with humor.