The hole in the basement of the Gibbs house was a portal to another dimension. It was created by an unknown entity and was used as a way to travel between dimensions. The hole was first discovered by Sam and Dean Winchester when they were investigating a case involving missing people. They later found out that the people had been taken to another dimension through the hole.
The Winchesters used the hole to travel to other dimensions on several occasions. Once, they went to a dimension where they encountered monsters called Reapers. Another time, they went to a dimension known as Purgatory, where creatures called Leviathans were trapped. They also used the hole to travel to Heaven and Hell.
At one point, the hole began collapsing and sucking in everything around it. Sam and Dean managed to close the hole using holy water from Heaven. However, it is possible that the hole could open again in the future.