Gravitons are quanta of gravitational wave (wave of the space time) just like photon are quanta of electromagnetic wave.
Being a quanta of a wave means that they are created and destroyed locally in a discrete fashion and then propagate as a wave. All waves are the sum of a finite integer number of quanta, but if it’s very large, it’s easier to just consider the wave rather than the quanta.
In the case of gravity, we are unable to make any experiment in which a single graviton as an effect because the effect of a single graviton are always to weak. The gravitational we can observe (and we have been able to only recently) comes from black hole collision and contain an extremely large number of gravitons.
Ordinary light is also an extremely large number of photons, but we are able to do single photon experiments.
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