填空 literally means 'to fill (填) a blank (空)' and functions as a verb meaning 'to fill in blanks' — typically in exercises, forms, or worksheets where missing information must be supplied. The character 填 conveys the action of inserting or stuffing something into a space, while 空 means 'empty' or 'blank', referring to the unfilled portion of text or a form. Together, they describe the common academic or administrative task of completing missing words, numbers, or phrases.

This term is widely used in educational contexts (e.g., language learning, exams, homework) and everyday situations like filling out registration forms or online surveys. It’s neutral in register and appropriate for both spoken and written Chinese. While often appearing as a verb phrase, 填空 can also function as a noun (e.g., 一道填空题 — 'a fill-in-the-blank question'), though its core grammatical role is verbal.