The travelers need energy to remove obstacles while exploring the Islands. The energy counter resets over time, but if you do not want to wait you can purchase energy sets for gems. You can get more energy from Fruit Bowls (a structure that can be demolished, which you can find on the Islands). Also, you can get energy as a reward for watching videos or even make it in Hermit's Camps, found on every Island save for the Island of Mysteries.