The "@:" command repeats the last command-line change (a command invoked with ":", for example :s/old/new/). You can move the cursor before using either of the repeat commands. Suppose you press dd to delete a line. Next, you might move the cursor, then press 5. (5 then dot). That will delete 5 lines. In normal mode, press J to join the next line onto the current line. Press . to join more lines. Or, you might use insert mode to type "hello ". Press Esc for normal mode, then move the cursor, and press . to insert "hello " again.