That is called a subpixel.
A) A pixel you see on your screen is in reality a group of 3 pixels of the primary colors R+G+B
B) We can have any shape of one color above other. Black shape over a white background.
C) We have the concept of aliasing, which is making pixels not entirely of one color, but smoothing into the surrounding color, this makes the image less jaggy.
D) The normal aliasing affects all three R+G+B colors at the same time.
E) Well, subpixel aliasing is the idea that we will make this gradients not only on the entire pixel, but simulating that we are smoothing across the pixel itself.
The what happens? That we have not the 3 R+G+B values equal, but we start reducing for example the Blue, then the Green and then the Red, which turns out as a rainbow of colors on the edges.
This is mainly done in typography in small size becouse each variation on the shape and weight of the lines counts. On bigger sizes it is not important.