The beacons were the warning-fires used by the men of Gondor to send word to their allies in Rohan that they were in need of aid.

The chain of seven beacon hills run between Minas Tirith and Rohan. From east to west they were: Amon Dîn, Eilenach, Nardol, Erelas, Min-Rimmon, Calenhad, and the Halifirien on the borders of the Eastfold.
Encyclopedia entry originally written by Lady~Eowyn