Discrimination means distinguishing between two or more people or things. Usually, discrimination means an unfair system that treats one group of people worse than another.
Discrimination isn't always a bad thing. You have to use discrimination to tell a good restaurant from a bad one, or a good idea from a bad one. However, this word is very tarnished by its main use these days: as a word for unfair treatment of people, based on racism, sexism, homophobia, or another prejudice. There is a huge history of discrimination in the United States.