I'm about to buy my first DSLR and I'm trying to understand every single aspect of it.

If I've got a prime lenses and I'm shooting a video with f/2.8 aperture, I'm going to have a fixed depth of field (1. nearer points where sharpness is acceptable, 2. actual focus point and 3. points further away in which sharpness is acceptable). So when I turn the focus ring what is happening to these three factors 1,2,3 mentioned above, technically? Is this fixed depth of field moving forward and backward linearly for the actor/object to be kept in focus while in movement?

I've been trying exhaustively to find this specific information here, on google, videos on youtube but I can't find the straight answer for my question.

Thanks