After your primary application is verified, schools send secondary applications, often to nearly every applicant regardless of competitiveness. Secondaries include school-specific essay prompts and typically a fee, usually somewhere around $100 per school.
The general rule of thumb is returning each secondary within about two weeks of receiving it. Pre-write responses to the common recurring prompts — diversity, a significant challenge, "why us," a gap year explanation — during the spring before they arrive, so you're not writing from scratch under a two-week clock for 20 schools at once. Secondaries are where you demonstrate genuine fit, so tailoring each one to that specific school's actual mission pays off more than reusing one generic answer everywhere.