When you read a book, you think of "places" in the book by page: the page where you stopped reading or the page number in an index. You don't have this convenience when you're editing files. Some files take up only a few lines, and you can see the whole file at once. But many files have hundreds of lines.
You can think of a file as text on a long roll of paper. The screen is a window of (usually) 24 lines of text on that long roll.
In insert mode, as you fill up the screen with text, you will end up typing on the bottom line of the screen. When you reach the end and press RETURN, the top line rolls out of sight, and a blank line appears on the bottom of the screen for new text. This is called scrolling.
In command mode, you can move through a file to see any text in it by scrolling the screen ahead or back. And, since cursor movements can be multiplied by numeric prefixes, you can move quickly to anywhere in your file.
There are vi commands to scroll forward and backward through the file by full and half screens:
Scroll forward one screen.
Scroll backward one screen.
Scroll forward half screen (down).
Scroll backward half screen (up).
(In the list of commands above, the ^ symbol represents the CTRL key. ...