Warning beacons of Gondor
The warning beacons of Gondor were an alarm system for the realm of Gondor.
The beacons were great fire places permanently manned by men of Gondor, and were placed on top of seven peaks in the range of the White Mountains. From east to west they were the Amon Dîn, Eilenach, Nardol, Erelas, Min-Rimmon, Calenhad and Halifirien (or Amon Anwar).
In the late Second Age and early Third Age, they mainly served to warn south Gondor of a danger for the northern province of Calenardhon, or vice versa, but after the Steward of Gondor Cirion granted Calenardhon to the Éothéod they were used mainly to warn the people in Anórien (the only part of Calenardhon Gondor kept in its realm) of danger.
The tomb of Elendil was hidden on the summit of Halifirien, westernmost of the beacon mountains.
for the movie version
One of the major changes made to the story by Peter Jackson's film The Lord of the Rings: The Return of the King is that when Gandalf and Pippin ride to Minas Tirith the beacons are not lit, since the despairing Denethor has decided not to send for help. In the movie there is a beacon just above the city, and to summon the Rohirrim Gandalf asks Pippin to evade the guards and light it. Once he has done so, the film depicts the other beacons being lit one by one until the last is sighted by Aragorn who is in Edoras, and it is this (and not the Red Arrow, which is not mentioned in the film) which finally helps him to persuade Théoden to muster the Rohirrim to Gondor's aid.
Another change is that in the movie there are about ten beacons as opposed to the seven described in the book.