The Gothic Era in the 12 Century started to develop in France and rapidly spread throughout Europe. The Gothic cathedrals are characterized by slender vertical piers, and counterbalancing buttresses and by vaulting and pointed arches.
A characteristic that defines Gothic Architecture is the Expansive area of the windows. Beautifully Decorated stained glass windows adorned the majority of these churches. Much unlike the it early Romanesque architecture, with it's mass and heaviness the Gothic style is differentiated by it's lightness and soaring spaces. With the introduction of flying buttresses, the reduction of the wall surface relieved part of their structural function. This allowed great windows to be placed into the churches, admitting light into the vast expanses of stained glass windows. The quality of light is an important element of the religious symbolism of Gothic cathedrals.
Another important detail that enabled the entrance of light into the churches were ribbed vaults.
The role of light that played in Gothic architecture was specifically to bring a holy presence into the church. With the light emanating through the stained windows gave the people a sense of stillness as if they were in the presence of holiness. Unlike previous churches which let little light in. The church wanted people to feel welcomed instead of frightened. And having more light illuminating these windows with bright and beautiful colors gave that calmness and tranquility.
so in conclusion, the role of light in Gothic architecture was basically to give an aura that made people have a more peaceful connection with the church.