The Tune of Echoes is the first melody you'll learn for the Harp of Ages. This tune is crucial as it enables you to activate Time Portals, allowing you to jump between the Present and Past versions of Labrynna. You'll learn this tune from Nayru (or rather, Veran possessing Nayru) after the events on Crescent Island and your first encounter with Veran.
To use the Tune of Echoes, stand on a Time Portal (marked by a swirling vortex on the ground), open the Harp of Ages menu with the Select button, and select the Tune of Echoes. Link will play the harp, and you'll be transported to the corresponding time period. Mastering the use of these portals is fundamental to solving environmental puzzles and accessing new areas throughout the game. Remember that changes made in the past can affect the present, and vice-versa.