Beneficial bacteria are the invisible nitrifying microbes that colonise your filter media and surfaces and do the actual work of the nitrogen cycle, turning ammonia into nitrite into nitrate.
They live mostly in the filter, not the water, which is why you should rinse filter media gently in old tank water — never under hot tap water, which kills them. Losing them causes a "mini-cycle" and an ammonia spike.
Bottled bacteria starters can seed a new tank, but the surest way to instant-cycle is to move mature media or décor from an established tank.