What is the cord under the hood of my 2005 F150 for?

Block heater. It is used to heat up the coolant so the engine warms up faster. Its useful for colder climates. Your truck was probably originally sold in the upper part of the Country where its advantageous to have it. More useful on a diesel but it works very nicely on a gas.

Looks like the previous owner installed a "block heater". If the truck is hard to start in the winter, you can plug it in at night to keep the engine block warm.

People usually thread these through the grille, or out underneath the bumper, for ease of access.

Its a heater cord for the winter time to keep the block warm . Its a $ 100 option .

Block heater, it replaces a freeze plug with a heating element and when plugged in to a standard 110v outlet, will heat your coolant either to a point that your engine starts very easily in winter, or is already blowing warm air when first started. I, like most other people, believe that most engine wear is caused by cold starts, the oil is cold and not flowing well and the more friction within, as well as more wear on the starter and battery to get it fired up. If your temp gets below 32, I would suggest plugging it in, just to save engine wear.