how does the person who "holds the place" for someone do that? What is the job? And for whom? I've often been confused by the term.

Taking somebody's place? When is a placeholder a lieutenant? How did Latin officium 'service, kindness, favor; duty' come to mean a room in which work gets done? Though we do have officeholders, too. Latin locus had a plethora of meanings: 'place, spot; seat, lodgings; (in war) post, station'. (See the entry in Lewis & Short.)

Also where does the British "leftenant" come from? "Lef"?

It's a complicated story, but basically, the pronunciation with f is older. See the entry in the OED1:


Ceci n'est pas un seing.