I've just started to learn from Tutorials, Unknown lines when extruded, how to disable the lines

How to disable these lines. and what are they indicating.

The screenshot isn’t very helpful to understand your problem.
Try posting a file.

You can upload a .3dm file by clicking on the vertical arrow icon above where you type a post, or by dragging the file into the are where you type a post.

That looks like something we see with buggy display drivers.
Open your V6 Notifications panel.
Is there a warning about old display drivers?