Nigger is a word which mostly refers to black people. Today, it is mostly used as a racist insult. Originally, it was simply used to refer to a black person. At that time, it was not considered racist.
The word came from a slang pronunciation of "negro", which is the word for the color black in Portuguese and Spanish. Writers such as Joseph Conrad, Mark Twain, and Charles Dickens used it. When they used it, they meant black person. The word was mostly used in the Southern part of the United States, where blacks were at one time held as slaves.
Today, it is an offensive and inflammatory racial slur in English, and is often considered to be hate speech. In the USA it's so taboo that it is not said by some people and it's instead called the "N-word".