Answer:
Countries document the entry and exit of people through their borders so that they may be able to plan their nations better be it security or economic wise. This is why they insist that foreigners coming in should be registered so that the government knows that they are there.
Undocumented workers are foreigners who come into a country for work but do not register themselves with the government. In other words, the government does not know that those people are in the country. Government policy is to find these workers and process them according to the law with some being deported and others finding ways to become documented.