Everything you see on your computer screen, no matter it is a picture or a vector graphic, even a text character, is simply a two dimensional array of pixels with specific colors. If you blow up the smile at the left top conner of below picture, the enlarged right side simle looks very blocky, each of these blocks is a pixel.
3D graphics will ultimately present with 2-dimensional array of pixels, how it works? In fact, the 3D graphics system produces the pixels with different colors that convince you to believe the scence you looking at is a 3D world than a 2D image.
Rasterisation is just the process to decompose the graphic objects into pixels. Below picture is a example of how the line is rasterized.
And below one is how a triangle is rasterized.
Below is the process of how a cow is drawn on the computer screen with OpenGL rendering pipeline. It's also an example to show that any 3D objects can be differentiate to tiny triangles, rasterize those triangles into pixels, then filling different colors to those pixels ultimately shape into the objects on the screen.
