ADR means "Automated Dialogue Replacement". Basically, it's when an actor or actors are called into a recording studio to either add background dialogue to a scene in a TV show or movie, or to record over an actor's lines in a TV show or movie to refine it and make it able to be heard by the audience. Here's an example of Hugh Jackman doing ADR for the movie Logan: https://www.youtube.com/watch?v=DyS5agWSIrY
It can also help when you think about it as the process through which many Japanese video games and anime get English language releases; voice actors are called in to record, or "dub", over the Japanese video to an English script.
Twice I've tried speaking to u. Twice u've answered only once. Twice u havent replied after i replied. Twice u've left chat. And twice u've kept me waiting thus, enabling my anxiety disorder to occur twice.