Uploaded video from SD card isn't as clear on Apple s Photos; how do I fix it/make it more clearer?

I took some videos on my Nikon camera and uploaded them to my iMac from my SD card. The video isn't as clear/HD as i wanted them to be and I thought it was just a problem with my camera, but my photos turn out to be very clear so I don't know the problem going on with my videos.

A video is a set of pictures taken very quickly and the camera is unable to process the images fast enough so it is lower resolution imaging in a movie versus a photo.
Only a 4K movie camera can get ultra-high resolution video.
Name the camera model and its capability can be looked up.
A higher speed SD card also helps.

HD videos need high write speeds to look good, I use Sandisk Extreme cards in my video cams and cameras.

Nikon cameras are not known for high video quality, sorry to say.