ADR is the process of re-recording dialogue, over the visually recorded scene. A process that has become much simpler over recent years thanks to the advancements of technology but has been used in film for countless years. In the years before digital recording technology this process was referred to as looping, because the scene would be physically spliced from tape and looped over and over for re-recording. Nowadays, the process is much faster and voice actors have an unlimited amount of opportunity to capture the perfect audio take.
ADR is usually implemented when there is a fault in the original audio’s take. These flaws can be anything from unwanted ambient noise (such as a plane flying overhead in the recording of a period piece) to unwanted audio from physical effects, such as a blowing fan, where the visual effect is wanted but the noise is not. Sometimes ADR can simply be the result of a creative change, such as the director deciding to change the voice of a certain character because it suits the themes of the film more.
A good example of ADR is the speeder chase in Star Wars episode II . In the recording of this scene, a large fan was used to emulate movement in the hair of the characters moving through a space in the stationary speeder, but this of course creates problems because the dialogue recorded on set is awash with the noise of a massive fan blowing. So by re-recording the audio, the creators were able to capture clean, coherent speech alongside immersive visual effects to create a credible, believable, immersive experience.
Jordan Vaughan: Creative Sound Design 