I don't speak Spanish, but I thought maybe an explanation of how I understand this term would be helpful. Excuse me if I'm stating what to you is obvious.

"brownfield" sites are potential sites for building in cities, which have already had buildings on them (as opposed to "greenfield" sites in the country). It's difficult to say without more context, but I imagine this phrase refers to an investment which would bring high returns through investing in developing the capacity (the potential) of brownfield sites.

Hope that helps!