What are braille letters?

I’ve heard of braille but exactly how does the system work?
You’ve seen braille on signs and perhaps even felt them and thought, how do people actually read this? Braille characters are formed using a combination of six raised dots arranged in 2 columns each with up to 3 dots, this is called a braille cell. The number of dots in each column and their location within the cell distinguishes one character from another. A braille reader will feel each cell, determine what cell represents, for example a letter or number, and move onto the next cell. It is critical that a braille dot is in precisely the right location within the cell, and is high enough so it can be easily read. The American Library of congress requires a braille dot to be .5mm in height.