A graviton is a term for a (theorized) quantum of gravity, analogous to photon, a quantum of EMR. You can find some writing that treats the graviton as just another particle, but it is certainly in the realm of theory. Theories of gravity that include the graviton are termed quantum gravity. Such theories have been devised and shown to be internally consistent as well as consistent with known laws of physics, and with observation, but they have not yielded practical tests, i.e., that might be carried out to disprove them or demonstrate some phenomena not otherwise explainable. There is a feeling that gravity must be a quantum phenomena in order to fit in with current theories of particles and forces (e.g., standard model). The notion that gravity can be geometry, per general relativity, and also a particle makes the field interesting. String theory includes gravity, with a sort of graviton.