Oh also it's possible that I missed that it's the last sentence you're wondering about.

With migration, they're talking about the mobility of labor. That the social relations of capitalism induces movement. They're describing a real observed phenomenon at the time, a move to factories and a movement between regions to find work. The question is, why do they do that? The commodification of labor means that it is worth less over time, that it can be slotted into your business processes with less training, that you can replace your workers more easily. On the other hand, workers must be employed to survive. They are landless. They have no farms. They are not apprentices.

Therefore, the desperation of the working class induces them to move to where they can find employment and employers can leverage labor commodification to get workers to move to their place of business. This mobility represents a coercive relationship between employer and worker established by capital relations and, in particular, labor commodification.

Relative surplus population refers to the reserve army, the people desperate for work.