Put simply, the Holocaust was the systematic genocide of over 6 million people, mostly Jews.
Part of Nazism was the hatred and intolerance of other religions, ethnic groups, and races considered "impure" by Nazi Germany. This hatred led to huge amounts of death and sorrow across Europe in the late '30s and '40s, with concentration camps being set up all over the place to facilitate the death of innocent people. You may have heard of the likes of Auschwitz or Dachau, just to name a few. These were camps the Nazis would send innocent people like Jews, Gypsies, Polish, and many others to be shot, gassed, starved, beaten, or worked to death. Even people that hadn't yet been sent to the camps were continually tormented and killed in pogroms. These were the continuous boycott and attempted extermination of things like Jewish business and places of worship. Pogroms also included mass shootings in the streets, as well as murder and other forms of assault.
The Holocaust continued right up until the war ended, with some camps continuing to murder people right up until they were liberated by Allied forces.
All in all, it was definitely one of the darkest times in human history, and something I hope will never happen again.
This was a rather basic overview of the Holocaust, if you want more information it isn't hard to find it, go ahead and just do some poking around online.