Okay, so imagine you're going to do some coloring or drawing. You have a book with lots of pages and each page has some pretty pictures already drawn on it. But there's also some blank spaces where you can draw your own pictures or color in the ones already there.
A black book of hours is kind of like that, but for grown-ups a long time ago. Instead of just pictures, there's also some words called prayers and other important things written in there. And instead of just coloring, the people who used these books would say the prayers and do other special things while looking at the pictures.
The reason they're called black books is because the pages would be made of special black material called vellum that was really expensive to make. The pictures and words would be written in fancy colors like gold and shiny reds and blues on top of the black.
These books were really important to people a long time ago because they helped them feel close to their religion and remind them of important things they needed to do. Nowadays we might not use these books as much, but they're still really cool to look at and admire all the fancy work that went into making them.