Some plants dispense pollen through a narrow terminal pore rather than splitting their anthers open. The pollen will not fall out on its own, and a bee that merely walks over the flower gets almost nothing. Bumblebees solve this by sonication: the bee clamps the anther in her mandibles, decouples her flight muscles from her wings and contracts them rapidly, producing a burst of vibration that travels into the anther and launches the grains onto her body.
The resulting buzz is audibly higher in pitch than the bee's flight hum, because the muscles are driven faster when they are not moving wings through air. Honey bees do not perform this behaviour, which is why glasshouse tomato growers rear bumblebee colonies rather than relying on hives.
Buzz pollination is not unique to bumblebees; carpenter bees and many solitary bees do it too. The frequent misstatement is that these crops cannot set fruit without bees at all. Greenhouse tomatoes were mechanically vibrated for decades before commercial bumblebees arrived, and self-pollination still occurs, just far less efficiently.