To fill out means to complete a form by writing or typing information in the blank spaces. When you fill out a permission slip for a field trip, you write your name, the date, and a parent's signature in the designated spots. When adults fill out job applications, they provide details about their experience and education. The phrase emphasizes completing all the required sections thoroughly.

A teacher might remind students to “fill out every question” on a worksheet, meaning don't skip any blanks.

You'll encounter forms throughout life: library card applications, contest entries, registration forms for summer camp, or medical forms at the doctor's office. Each form has specific spaces where particular information belongs. Filling them out carefully matters because incomplete or messy forms can get rejected or cause delays.

People sometimes confuse “fill out” with “fill in,” and they're almost the same. “Fill in the blank” usually refers to a single space, while “fill out the form” suggests completing an entire document with multiple sections.