I'm not sure if the same definition applies in English, but a "marginal" is a person who has been left "marginalized" from society, like those in extreme poverty and are neglected by the government.
There is also a more literal interpretation of it considering that the extremely poor might often build their hovels on the "margins" of a body of water unattractive for development and that might be prone to floods/landslides. Also, since bridges are often built over rivers, homeless people will live under the bridges, making themselves "marginal" dwellers.
The same applies for the sides of highways, where it is often illegal to settle, but that impoverished people might do so because it is the only unoccupied place where they can live (illegally) close to an urban area.
