There seems to be this reoccurring phrase to describe a specific type of person in the suttas. For example in MN 45
"And what is the taking on of a practice that is painful in the present and yields pain in the future?"
"There is the case where someone is a cloth-less1 ascetic, rejecting conventions, licking his hands, not coming when asked, not staying when asked. He doesn't consent to food brought to him or food dedicated to him or to an invitation to a meal. He accepts nothing from the mouth of a pot or from the mouth of a bowl. He accepts nothing from across a stick, across a pestle, from two eating together, from a pregnant woman, from a nursing woman, from a woman lying with a man, from a food collection, from where a dog is waiting or flies are buzzing. He takes no fish or meat. He drinks no liquor, wine, or fermented drink. He limits himself to one house & one morsel a day, or two houses & two morsels... seven houses & seven morsels. He lives on one saucerful a day, two... seven saucerfuls a day. He takes food once a day, once every two days... once every seven days, and so on up to a fortnight, devoted to regulating his intake of food. He is an eater of greens, millet, wild rice, hide-parings, moss, rice bran, rice-scum, sesame flour, grass, or cow dung. He lives on forest roots & berries. He feeds on fallen fruits. He wears hemp, canvas, shrouds, refuse rags, tree bark, antelope hide, strips of antelope hide, kusa-grass garments, bark garments, wood-shaving garments, head-hair garments, animal wool, owl's wings. He is a hair-&-beard puller, one devoted to the practice of pulling out his hair & beard. He is a stander, one who rejects seats. He is a hands-around-the-knees sitter, one devoted to the exertion of sitting with his hands around his knees. He is a spike-mattresser, one who makes his bed on a bed of spikes. He is a third-time-in-the-evening bather, one who stays devoted to the practice of bathing in water. Thus in a variety of ways he stays devoted to the practice of tormenting & afflicting the body. With the break-up of the body, after death, he goes to a bad bourn, destitution, the realm of the hungry shades, hell."
I really don't understand what is meant by this. Can anyone please give a simple explanation or some reference to other translation or commentaries?