It means that they are illegal immigrants.
Mexico is the country that has a large border with the United States, so it has become a route of illegal immigrants from various countries trying to enter the United States.
However, the tightening of immigration laws and the dangers of illegal crossing make many of these immigrants give up and remain in Mexico itself. In desperation to survive, these immigrants who can not cross the border accept subaltern underemployment in the cities of Mexico, that is, Mexico is also a country that receives illegal immigrants.
