Afterload is the pressure the heart has to work against to push blood flow out of a chamber.

Aortic stenosis is an example of increased left ventricular afterload due to stenotic vessels; imagine how hard you have to push through the hard vessel to get the the blood out.

Same thing with idiopathic pulmonary hypertension. IPH has increase pressures in the pulmonary system, which causes increased work in the RV to push against the increased pressures. This leads to back up of blood into the RA--> then venous system-- >increased pressure in venous= edema.