From the Sweepfinder2 publication:

B-values range from 0 to 1 and are proportional to local reductions in genetic diversity or effective population size due to background selection. McVicker et al. (2009) provide a method for inferring B-values using comparative data, thereby providing an opportunity for separating background selection from the effect of selective sweeps inferred from within-population polymorphism data.

McVicker 2009: https://journals.plos.org/plosgenetics/article/info%3Adoi%2F10.1371%2Fjournal.pgen.1000471

You'll have to read the methods section in that paper and do some lifting on your own. Or email the authors of Sweepfinder2.