What type of film technique is “blurring” called?

by j l t

Question by Faye: What type of film technique is “blurring” called?
In a video/movie, the object in the background is blurred, making the focus on the object in the foreground. Then, the blurring changes and the object in the foreground is blurred with the focus on the object in the background. This technique is in most of the videos and movies I see. What is this technique called?
I would include an example, but most movies and videos have this technique.
Please help me 🙂

Best answer:

Answer by Robask
fade in?

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