I don't think it's really an idiom. What I think she's saying is that one of the DVDs in her collection is very different in content and style from all of the others.
For example, if she had in her collection Four Weddings and a Funeral; Love, Actually; Attack of the Killer Tomatoes and Notting Hill, three of them are romantic comedies and the fourth is a horror movie. The horror movie is not like the others.
Normally, the phrase you would use to tell people to find the item in a list that is different from the others is: "Find the odd one out". But this person was quoting from the popular children's TV series Sesame Street. Some objects would be lined up -- say, an apple, an orange, a telephone and a banana -- and the children had to guess which item was the odd one out. To give the children time to think, the characters would sing a song:
One of these things is not like the others,
One of these things just doesn't belong,
Can you tell which thing is not like the others
By the time I finish my song?
Student or Learner