This is a Jewish prayer recited on Rosh Hashanah and Yom Kippur. It is considered a central part of the service.
The story behind it is that about 1000 years ago, the Bishop of Mainz strongly insisted that his good friend and advisor, Rabbi Amnon, converted to Christianity. The Rabbi didn't want to do this, of course, but the Bishop was being very forceful. So he asked for a few days to think about it.
When he got home, he was devastated that he had even given the impression that he would forsake his Judaism and spent the 3 days praying for forgiveness and didn't return to the Bishop.
The Bishop ordered him brought before him, and Rabbi Amnon said that his tongue should be cut out for even giving the impression he would consider becoming Christian. Instead, the Bishop said that it wasn't his tongue, but that his legs should be removed for not coming as he had promised. The Bishop had his hands and feet cut off, joint by joint, asking him after each amputation if he would convert. Every time, Rabbi Amnon said he wouldn't. Eventually, the Bishop had him carried home, maimed and mutilated. Three days later was Rosh Hashanah, and Rabbi Amnon asked to be brought before the Aron HaKodesh before the Kedushah in the Amidah, one of the central portions of the service. He recited this prayer and died.
A few days later, Rabbi Amnon appeared to Rabbi Kalonymus in a dream and taught him the prayer, and asked him to distribute it to all Jewry. This was done, and we recite the prayer, in the same place, on Rosh Hashanah and Yom Kippur.
The first few words are "Unetaneh Tokef Kedushat HaYom" - "Let us now recite the holiness of the day". It acknowledges G-D's omniscience and his perfect recall of everything good and bad that we have done throughout the year. The image is created of the Book of Life being open, with everybody's name being written in it (we hope). We imagine ourselves as a line of people passing in front of G-D to be judged - "as a shepherd counts his sheep".
The second paragraph starts "B'Rosh Hashanah Yikateivun, u'v'Yom Zom Kippur Yechatemun" - "On Rosh Hashanah it is written, and on Yom Kippur it is sealed" - where "it" is our destiny for the year. It then goes on to describe what might happen to us, including...
- How many will be born
- How many will die
- Who will live
- Who won't
- Who (will die through) fire
- Who (will die through) water
- Who will be rich
- Who will be poor
And many others.
Then at the end, the whole congregation calls out together "U'teshuvah, u'tefillah, u'tzedakah ma'avirin et ro'a ha-gezeirah" - "Repentance, prayer, and righteousness remove the evil decree". Having just spoken about all the bad things that could happen, we can put things right in this way. Many prayer books have in small letters above this "Tzom, Kol, Mamon" - "Fast, Voice, Money" - indicating that this is what we should actually do.