Antisemitism is prejudice against and hatred for Jewish people. Discriminating against Jews is an example of antisemitism.
The Jews are also known as Semitic people, and the culture and qualities of Jews are known as Semitism. Place the prefix anti- (meaning "against") in front of semitism, and you get a noun describing actions taken against Jews. Antisemitism is an ugly thing, similar to racism or sexism. Unfortunately, antisemitism has been one of the strongest forms of hatred in the history of the world, as seen in events like the Holocaust, in which Nazi Germany killed six millions Jews.