Copied from http://kippenjungle.nl/basisEN.htm#kipkleurEN -
"Partridge/Brown" symbol eb, the most recessive form hence lower case e. The males are wildtype looking including groundcolor wing triangle, but they have heavier hackle and saddle striping. Their underfluff is often dark. The hens are wildtype except they lack the salmon breast; it is stippled instead. Sometimes black animals are based on this partridge/brown color. They have light underfluff and can have nice yellow legs (by another gene). This requires multiple, partly unknown, additional melanizers. This color is also difficult to set in the chicken calculator. Set both "Melanotic" and "Charcoal" to "melanized". Typical for this kind of black is that "Columbian"-like restrictors can turn the black breast (and wingbows) to groundcolor, while the more dominant E-alleles described above do not allow this;
Basically it's a coloration that makes hens have patterning through the chest.