The endless knot or eternal knot is a symbolic knot and one of the Eight Auspicious Symbols. It is an important cultural marker in places significantly influenced by Tibetan Buddhism.
One common form of the Endless Knot
The endless knot has been described as "an ancient symbol representing the interweaving of the Spiritual path, the flowing of Time and Movement within That Which is Eternal. All existence, it says, is bound by time and change, yet ultimately rests serenely within the Divine and the Eternal." Various interpretations of the symbol include (among others):
I've taken this from the Wikipedia article on the Endless Knot (and left out the bits I didn't understand!)
The endless knot is also sometimes found in Chinese art and used in Chinese knots.