Its called, appropriately enough the tailskid, and it is, as
aloha stated to protect the rear fuselage during an overrotation on takeoff or landing.
Electrically powered on the -100 series, hydraulically on the-200 it operates in conjunction with the landing gear.
If an overrotation does occur it will be noticed during a walkaround inspection by the presence of scrape marks, or in a more severe impact there is a crushable 'cartridge' attached that will be, well, crushed.
That would require a more thorough inspection by maintenance.The tailskid itself does not PREVENT an overrotation, the physics of a 160,000lb aircraft acting
against a 'puny' tailskid are obvious.
However all modern jet aircraft have to prove in certification called VMU tests that they can be rotated
to such a high pitch attitude on take-off that the tail actually scrapes along the ground until the aircraft lifts
off. You may have seen pictures of this, where the test
aircraft has sparks and flames visible from the tail
the manufacturer will usually mount a wooden skid to minimize any damage.
Aircraft therefore are geometrically limited from reaching
a pitch attitude where they cannot liftoff, or accelerate
as happened on several early Comet accidents where the aircraft was overrotated to such an attitude and indeed, crashed due to its inability to climb or accelerate.
The best contribution to safety is a competent Pilot.