It looks like a regular bubble, and is kept in a glass container. It was one of Atlantis's most prized possessions, being one million years old.
As Lord Royal Highness explained, this bubble was brought here when the first Atlanteans colonized this place.
Role in episode
It was what SpongeBob and Patrick wanted to see when coming to Atlantis. Patrick takes a picture of the bubble and it then pops. They later find out that that wasn't the actual bubble and was a replica. Patrick takes a picture of the real bubble and it pops. The bubble is later replaced with Plankton.