Command mode is used for cursor movement, editing, retrieving and saving files, etc.
If you want to type in text, you need to be in Insert mode . Once in insert mode, you can type in your text. Use Backspace to delete to the left of the cursor. If you need to move the cursor to another line, or make a change, or pretty much anything else, you need to press ESC to get back to Command mode first. Here is a list of the most common vi commands.