The word 前后 is a coordinate compound formed by combining two antonyms, 前 (front/before) and 后 (back/after), into a single lexical unit. This structure, common in Chinese, creates a new meaning that encompasses the entire range or totality implied by the two opposites. In this case, it abstracts the spatial or temporal concepts of "front and back" to express the idea of "around" a time, "throughout" a period, or the entirety of a sequence, thus functioning as a noun or adverb meaning "from beginning to end", "about", or "in front and behind".