Proskynetarion is a special book used in Orthodox Christian churches. It has all the songs that people sing during a special religious ceremony called Vespers.
During Vespers, people gather and sing religious songs to worship and praise God. The Proskynetarion helps them remember all the songs they need to sing.
It is a very important book because it helps people sing together and worship together. Without the Proskynetarion, people might forget which songs to sing, and the worship service might not go as smoothly.
Think of the Proskynetarion as a big songbook that people use during church. Just like we use a songbook during karaoke to remember the words to songs, the Proskynetarion helps people remember the religious songs they need to sing during Vespers.