Orbs are left behind after a Sim who's completed their Life Dreams has died. Once you have picked up an orb, you can choose to either sell it, pass it on to another Sim to upgrade it, or to build a place.
Each orb has a different value, depending on how upgraded it is. Bronze Orbs sell for 40,000 Simoleons, Silver Orbs sell for 90,000 Simoleons, Gold Orbs sell for 180,000 Simoleons, and Platinum Orbs sell for 300,000 Simoleons!
Using Orbs to build areas
Certain types of Orbs are required to build different areas, mostly on the Mysterious Island.
Ice Ape Arcade
The Ice Ape 'Arcade' itself costs 1 Bronze Geeky Orb & 1 Silver Geeky Orb. To unlock the Skill Tester Hobby, you must pay 6 Silver Geeky Orbs and 3 Gold Geeky Orbs. To unlock the Pinball Machine Hobby, you must pay 5 Geeky Gold Orbs and 1 Platinum Geeky Orb.
The Public 'Beach' itself costs 2 Bronze Sporty Orbs and 2 Silver Sporty Orbs. To build the Swimming Kiosk, you have to pay 3 Bronze Sporty Orbs and 1 Silver Sporty Orb. To build the Surfing Kiosk you must pay 4 Bronze Sporty Orbs and 1 Silver Sporty Orb. And in order to build the Snorkel Hire Kiosk, you need to pay 8 Bronze Sporty Orbs and 4 Silver Sporty Orbs.
High Top Carnival
The High Top Carnival itself costs 5 Bronze Entertainer Orbs and 2 Silver Entertainer Orbs. To unlock the High Striker Hobby you need to pay 2 Silver Entertainer Orbs and 4 Gold Entertainer Orbs. To build the Jumping Castle you must pay 3 Gold Entertainer Orbs and 1 Platinum Orb. In order to build the Carousel you have to pay 3 Gold Entertainer Orbs and 1 Platinum Orb. And to build the Ferris Wheel you need to pay 3 Gold Entertainer Orbs and 2 Platinum Orbs.
Barkshire Pet Park
The Barkshire 'Pet Park' itself costs 2 Bronze Animal Fanatic Orbs and 4 Silver Animal Fanatic Orbs. In order to unlock the Dog Agility Hobby you need to spend 3 Silver Animal Fanatic Orbs and 2 Bronze Animal Fanatic Orbs. To build the Nine Lives Cafe you have to pay 2 Silver Animal Fanatic Orbs and 3 Gold Animal Fanatic Orbs. And lastly, to build the Cat Show Pavilion, you need to spend 3 Gold Animal Fanatic Orbs and 1 Platinum Animal Fanatic Orb.
Passing on Orbs
Passing Orbs on to other Sims is the only way to upgrade orbs. Your first Sim will get a Bronze Orb, giving it to another Sim now gives that second Sim the chance to get it up to a Silver, then a third Sim can get it to a Gold and a fourth Sim to a Platinum.