It is the multiplier that accuracy gets multiplied with when calculating if it hits. More than 1 means it's easier to hit, less is harder. Only applies to small arms.
Technically, it applies to everything, but only really has an effect for small arms against infantry or AT weapons against vehicles in practice as larger weapons have such a low chance to hit against infantry that they largely depend almost entirely on their rounds' AoE scattering to their intended targets anyway while small arms' accuracy multiplied against vehicles' target sizes are always guaranteed to hit.