Rubrication is like coloring inside the lines when you are coloring with your crayons. Think of a coloring book where there are black lines that make the picture and you use your favorite colors to fill in the spaces between the lines. Rubrication is when people who wrote books a long, long time ago did the same thing but with words instead of pictures.
They used a special ink that was usually red or sometimes blue, and they would make the titles and headings stand out by coloring them in with that ink. This way, it was easier for people who read the book to find important parts because they would be able to spot them right away.
Rubrication was especially important before the printing press was invented because books were made one at a time by hand, so they were very expensive. Making the titles and headings stand out with rubrication made the book look nicer and more important, so people were willing to pay more for it. Nowadays, books are printed in large numbers, so rubrication is not used as much, but it was really popular in the olden days.