If you look down at an offset press from above, you’ll see four ink fountains: one for each color of ink. Each of these fountains sits at the top of an assembly containing a cylinder that will hold the printing plate and one that will hold the rubber blanket.
Each fountain also has a reservoir of a water-based liquid known as fountain solution. This water solution is rolled onto the printing plate to repel the ink from the areas that will become white space on paper.
The roller distributes ink onto the printing plate in an even, controlled manner, creating consistent ink coverage for the entire plate.