Some commands interpret the argument as a repetition count. For example, giving an argument of ten to the key C-f moves forward ten characters instead of one. With these commands, no argument is equivalent to an argument of one.
Negative arguments tell most such commands to move or act in the opposite direction.
With C-u , you can type the argument digits without holding down shift keys. To type a negative argument, start with a minus sign. Just a minus sign normally means -1. C-u works on all terminals. It multiplies the argument for the next command by four.