PA 873 begins at an intersection with PA 309 in the community of Schnecksville in North Whitehall Township, Lehigh County. From this intersection, the route heads north on a two-lane undivided road, passing a mix of homes and businesses. The road turns northeast and passes residences and fields before intersecting the western terminus of PA 329. From here, PA 873 continues north into the residential community of Neffs before curving northwest and entering Washington Township. In this area, the road passes through a mix of housing developments and farmland as it turns north and passes over I-476 (Pennsylvania Turnpike Northeast Extension). The route continues through rural areas of homes and woods prior to crossing into Slatington. Here, PA 873 becomes Main Street and is lined with residences before reaching the commercial downtown, where the road makes a sharp turn to the east. The route turns north onto Walnut Street and passes more businesses before passing a few homes a short distance to the west of Slatington Airport. PA 873 leaves Slatington for Washington Township again and runs through wooded areas with some residences. The route turns northeast and crosses the Lehigh River into Lehigh Township in Northampton County, where it passes over Norfolk Southern's Lehigh Line before ending at PA 248 at the base of Blue Mountain.