Magic that is characterized as 'low' is typically aimed at ends attuned to the baser instincts, such as sex, or other pleasures; the pursuit of stability or success in some personal or social sphere such as in one's career or romance; or geared toward changing one's fortunes in the court room, or in speculation, games or chance, or other gambling ventures. Low magic attempts to obtain or produce the personal satisfaction of the witch or sorcerer, often by influencing or manipulating the conventional or mundane world in selfish and materialistic ways through traditional spellcasting techniques.
Folk magic specializes in the techniques of low magic, to a degree, featuring as it does formulaic means of obtaining money, jobs, sex, love, or other ends common in the mundane world. It is instructed for and adaptable to everyday life, and often to a context where there is no short supply of the stuff that makes it work successfully.
Some who employ the phrase often do so as a means of disdaining low magic and putting its practice down as at best a waste of time, or at worst an endangerment to one's immortal soul.
One might also see it as pertaining to the lowest levels on such conventional symbolic structures as the Tree of Life, the yogic chakras, or even Maslow's Hierarchy of Needs.