Often used in politics, to "win by a landslide" means to have a blowout or overwhelming victory. A presidential candidate capturing 60 percent of the vote would likely be said to have won by a landslide.
"Win by a landslide" is also used in sports, where one team defeats another by a significant amount. While this expression is generally used for large victories, it actually originates from the propensity of undecided voters to jump on the bandwagon with the popular choice. As with a physical landslide, the effect causes building momentum. In a sporting event, a team often pulls away when the opponent gets demoralized.