Philosophically Paolo Freire wrote that oppressors deny the humanity of the oppressed by convincing them that they are merely acted upon by forces outside their control. Base building is the process of undoing this harm by helping oppressed people realize their collective power to affect the world and thereby reclaim themselves as fully human.
Materially Base builders work with people oppressed by capitalism, racism, imperialism, and many other intersecting forces to build long-term structures of communal power that fight this oppression. This can take many forms: labor or tenant unions, mutual aid networks, educational systems, and so on. The key is to get oppressed people to work together, to own and lead the means of their own liberation.
Strategically Revolutionary change is necessary to dismantle the oppressive structures that rule our lives and mitigate the harm they’ve already done to our planet, and in order to succeed in truly liberating the masses, a revolution must be an expression of their power. As Freire indicates, this is only possible if they understand themselves to have power that can be wielded for the good of all. Base building creates that understanding in the most direct way possible: by helping oppressed people wield their collective power to improve their lives.