Provided you are already in 3rd person mode, this command will toggle the visibility of the first person arms that usually appear on the screen in first person. This means when you, for example, punch, both your third person model and the hands (first person) would punch.
show1stperson
Show1stPerson
The above command will show the first person hands on the screen, even though you aren't in first person mode.
s1st
The s1st command is an alias for Show1stPerson (shorter version) - it has the exact same function.