Do you mean by "jumper wire" one from the green grounding terminal to the
white neutral terminal on a grounding receptacle? It's a cheater. As long
as whoever did it didn't connect it to the hot (supposed to be black)
wire it's OK, sort of, as long as there is no fault in the neutral wire.
The 2-socket polarized receptacles are getting hard to find at hardware
stores, so people will replace worn out ones with a grounding type
receptacle.
GFIs do not solve missing or poor grounding problems.
A GFI is not a substitute for a bad ground. It is not a substitute for
replacing 2-wire no ground romex unless you run a separate ground wire
parallel to the cable. Also much of that 2-wire is not plastic sheathed
romex but textile braid which, I have read, is less safe than knob&tube.
For one thing rodents like to eat the braid off the wires.
It turns the power off to the GFI receptacle and all receptacles
connected "downstream" from the GFI when the current does not match
between the two conductors (hot and neutral) within a small amount, 5 ma
I think, which is barely enough for us to perceive and not dangerous to
ordinarily healthy persons.
"Installed exactly right" includes being properly grounded.
Refrigerators and other large appliances often have enough leakage to
trip a GFI. The grounding plug and receptacle are supposed to provide
protection. My 1958 GE refrigerator doesn't even have a polarized plug.
Since it's on the opposite side from the sink it's not as much of a risk.
Al