What does this warning mean: "May contain traces of..."?

What does this warning mean: "May contain traces of..."?

Although there is a legal requirement to list allergens, a warning of possible unintended contact between the product and these allergens does not have to be indicated on the product label. For example, if a plain milk chocolate bar is produced directly after a chocolate bar containing nuts, traces of nuts may be transferred to the plain milk chocolate unintentionally. To cover cases such as this, manufacturers often use the warning, "May contain traces of..." However, there is no legal obligation to place this warning on packaging.