Hi Rahim,
did you watch the entire lesson? Kent mentions blurriness due to resolution.
If you use the default Sculpting File, you get a sphere with rather low resolution (you can see the Resolution when you enable Wireframe, or switch to Edit Mode):

You can switch to Object Mode and add a Subdivision Surface Modifier of Level 2 to the sphere and Apply it. Then go back to Sculpt Mode and try again.
(There are other ways, but this is probably what Kent did here…)