Pruning (as in "pruning a tree") is a colloquial expression for a form of male genital modification similar to trumpeting, glansectomy, and glans sculpting. To put it simply, most, or all, of the glans is cut away (pruned) in order to give a more desirable shape.
Some sensation is, of course, lost, but at the same time, sensitive tissue and nerve structures are also exposed. Some people have also reported that removing part of the erectile tissue (or partial penectomy) results in firmer erections as well.
It should, of course, be noted that this procedure is incredibly bloody, and there is the potential for infections inside the penis that could do permanent harm. That said, because of the good blood supply and the lack of foreign materials as you'd have with piercings and procedures like genital beading, pruning tends to heal quite quickly.