Okay, kiddo, let me tell you about the Book of Common Prayer from a long time ago.
The Book of Common Prayer is basically a big book filled with prayers and religious readings that people in the church used during their services. It was created a really long time ago in England and has been updated and changed over the years.
The version you're asking about is a special one that was made in 1845. This version is called the "illuminated" version because it has really pretty pictures and designs that were painted by artists.
The people who made this version wanted it to be extra special and beautiful, so they used special techniques to make the pictures really stand out. They used things like gold, silver, and different colors to make the pictures look amazing.
So, in summary, the Book of Common Prayer is a big book of religious readings and prayers that people used in church, and the 1845 illuminated version is a special one with really pretty pictures that were painted by talented artists using special techniques.