These are called homogeneous coordinates, and as expected since you have 4x4 matrices, vectors are also 4D, usually in the form (x,y,z,1). The idea is that you

These are called homogeneous coordinates, and as expected since you have 4x4 matrices, vectors are also 4D, usually in the form (x,y,z,1). The idea is that you can multiply each coordinate by a non-zero scalar and the vector represents the same point, it has several interesting mathematical properties. For computer graphics, the main advantage is that you can represent translation and projection in the matrix in addition to the other linear transformations like rotation and scaling. For projection, the 4th vector coordinate, usually named "w", is called the homogeneous coordinate and it is typically not 1 after going through the projection matrix. As a final step, it divides all the other coordinates, so (x,y,z,w) becomes (x/w,y/w,z/w,1), x/w and y/w are the 2D screen coordinates, and z/w goes into the Z-buffer.