Indra's Net is a metaphor taken from the Avatamsaka (Flower Garland) Sutra, an important Mahayana Buddhist sutra.
The sutra describes a vast net that reaches infinitely in all directions, and in the net are an infinite number of jewels. Each individual jewel reflects all of the other jewels, and the reflected jewels also reflect all of the other jewels.
The metaphor illustrates the interpenetration of all phenomena. Everything contains everything else. At the same time, each individual thing is not hindered by or confused with all the other individual things.