Chol is the opposite of qodesh (holy, set apart). It describes what is common, everyday, not consecrated to God. The distinction between holy (qodesh) and common (chol) is one of the most fundamental categories in Levitical law and prophetic theology. Priests were especially responsible for teaching Israel this difference.
The qodesh/chol distinction is not about things being evil or good — common things are not sinful. The distinction is about belonging. What is holy belongs to God; what is common belongs to ordinary human use. Priests who confused these categories — treating the holy as common — committed grave sin (as Nadab and Abihu learned). The NT expands the category: all believers are a "holy priesthood" (1 Peter 2:9), meaning everyday life is called into the sphere of the sacred. The mundane is sanctified through Christ's lordship over all of life.