It's not hard. There's this cup that gives you a wish of limitless power. To get this cup, you must kill other people who are also trying to get the cup. Each person is paired with a famous historical figure of much renown, who are usually super-powered. The idea is that once you kill everyone else, you win. You also get three command seals in the form of some kind of tattoo. These mark you as a "master" of a particular servant and allow you to control them. You can expend a seal to achieve previously unattainable feats such as teleporting somewhere or being granted a boost in stats. Stats come in the form of Strength, Endurance, Agility, Magic and Luck, whilst servants also get a "special ability" of some kind. It's basically their super attack.
Also, we're being added as an extra master/servant duo to this "war" as such. Normally, there are only seven masters and servants, but we're the 8th for plot reasons.
Also, this "cup" is called the Holy Grail and we're fighting in the Holy Grail War. Check out the anime, it's pretty dope. It starts with fate/stay night.