Orbs are energy sources used by Avalarians to power their portals, along with talismans. You receive Orbs from doing random jobs for the people who live in that world, or finding them hidden away somewhere. Although talismans are the first thing Spyro was told to collect, it turned out that orbs are even more important to the story. The Professor can use the Orbs to seal off Ripto. Not only did they break the seal on the castle door (along with other locks across Avalar), they gave Spyro powerful abilities for a limited period of time. Spyro used this to his advantage when facing Ripto. You will need orbs to unlock Ocean Speedway in Summer Forest, Metro Speedway and Icy Speedway in Autumn Plains and Canyon Speedway in Winter Tundra. You will also need 40 orbs to fight the final boss, Ripto.
With 64 orbs in all, they are the least abundant of the original collectibles, along with, again, talismans.