While you are editing a file, you can move through it one line at a time, several lines at a time, or screens at a time. You can use these commands to move up and down through a file:

Command

Moves the cursor:

<Ctrl-D>

Down (or forward) half a screen. The cursor stays where it is -- the text moves underneath it.

<Ctrl-F>

Down (or forward) almost a full screen. This lets you move forward through the file very rapidly.

<Ctrl-U>

Up (or backwards) half a screen.

<Ctrl-B>

Up (or backwards) almost a full screen.

If you move forward far enough through vitest, you will see a number of lines that are blank except for a tilde (~) as the first character. These lines are actually beyond the end of the file -- the file ends with the line:

And the mome raths outgrabe.

vi could just show an empty screen after this last line, but then you would not know if the screen was empty because you had reached the end of the file or if the file just contained a lot of blank lines; therefore, vi uses ~ to mark lines that are past the end of the file.