In a nutshell:
Think retro, then subtract 1500 years. This movement takes broad influences from the Middle Ages (5th-15 Century) known as the Medieval period, while also taking influence from the following Renaissance period. It encompasses other movements in these periods such as Early Christian art, Migration Period art, Byzantine art, Insular art, Pre-Romanesque, Romanesque art, and Gothic art, as well as many other sub-categories within these central styles.
This being said, the movement is less defined by the exact period and more by the blending of historic influences with modern graphic typefaces and styles. It uses the contrast of styles as a tool to disrupt the viewer’s expectations. The result is a timeless and authentic feel.