It's a variety of the "Celtic knot", a common artistic pattern which predates Christian contact with the Celts. After the arrival of Christianity in the 5th century, it was adopted into all sorts of mediums- such as the decoration biblical manuscripts. For example the Lindisfarne Gospels contain many variations of these 'infinite' knots in their illustrations.

As to their pre-Christian meaning, that's up for debate. It's generally accepted that they represent some sort of 'beginning and end' or 'infinite regression' theme. There are all sorts of interpretations. Some historians claim that the Celts did not represent human figures in their art, leading to a purely geometric aesthetic. It's unclear to what extent that played a role.

Today, it's popularly called the 'Celtic love knot'. These patterns have commonly been adopted into pieces of jewelery, such as weddings bands, and even your necklace. The idea is that if the knot represents eternity, then the joining of two people connected by this design will surely be eternal.