A boom downhaul is one of the three possible control lines for the three sides of the mainsail. The other two are the boom vang (controls leech tension) and the mainsail outhaul (controls foot tension). Catalina 22 and similar sailboats have a  short length of line attached to the bottom of the gooseneck car on the boom that ties off to a cleat at the base of the mast as shown below.  It simply keeps the boom from sliding up past a certain point on the mast after you hoist the mainsail. It gets the job done but it isn’t easy to adjust, especially while under sail.