It means settlers, as opposed to indigenous peoples.

This is from the book (fittingly titled) Settlers.

Eta: I should have included how the people the euro anericans enslaved fit into this, as it is central to the author's point:

In general, it's commonly agreed that Euro-Amerikan workers earned at least twice what their British kinfolk made — some reports say the earnings gap was five or six times what Swedish or Danish workers earned.(34) Even a whole century later, the difference was still so large that Marx commented:

"Now, all of you know that the average wages of the American agricultural laborer amount to more than double that of the English agricultural laborer, although the prices of agricultural produce are lower in the United States than in the United Kingdom... "(35)

It was only possible for settler society to afford this best-paid, most bourgeoisified white workforce because they had also obtained the least-paid, most proletarian Afrikan colony to support it.

It's an attempt to break through the bullshit narrative that America and Europe are somehow separate historical processes.