So i was watching some camera follow tutorials in udemy but in that the person who was teaching got a class named CamerFollow but in my visual studio it's not available i can"t understand . it was like private CameraFollow camerafollow = null;
Most likely he wrote that script. Go back and check wherever Udemy provides sample code. If he did not, perhaps it is found in the Unity Standard Assets, but if so they should call it out in their class notes. Otherwise you can get various CameraFollow scripts all over the net, but each one is special and different from the others, and they are certainly NOT fully interchangeable.
For camera stuff specifically, you might also want to look into Cinemachine (which is a free package provided by Unity that you can add to your project.) It basically does pretty much everything you'd ever want to do with a camera without forcing you to write boilerplate code first, and it works both in 2D and 3D projects.
I agree completely with @Hendrik-Mans about Cinemachine, but in the narrow case of wanting to complete the specific Udemy tutorial, you need to find out (from that tutorial) where that particular script (CameraFollow) came from. It's also possible you are doing a tutorial that is part of a larger series of tutorials, and they develop the CameraFollow script in earlier episodes. But in any case, you're likely to need the exact one they want, if you want to use it unmodified in that tutorial. When you understand that, go check out some of Unity's tutorials on Cinemachine. It's worth having it in your toolkit.