Definition

Replay value is how fun a game is after you’ve beaten it - either by starting a new game or by continuing the game after you’ve completed the main objective of it.

Game developers has increased the replayability value of games in several ways:

Scoring Points
Speed Run
Adding branches in game to make it less linear , so you can experience different things in the same game.
Including a Karma meter so that every choice a player makes influences the game in different way
In RPGs and games with RPG Elements, having a number of different possible ways to build your main character or characters, such specializing in weapons or magic
Including some for of New Game Plus
Including several bonus features, such as bonus bossess or bonus dungeons
Great Music
Generating certain things, such as levels, procedurally.
Adding difficulty levels
Including a map editor and other means for players to generate and share their own content
Varied enemies/environments that require different skills to pass, or even different styles of play
Multiple Endings
And just creating a good game in general