I don't believe this question is intended for the gap between high school graduation and the start of your undergrad. Here's a good example:
As an active duty military service member 1 year into your B.S. program, you receive orders for a 6 month overseas deployment. Your transcript shows steady coursework up until your 6 month gap, followed by continued coursework upon your return. For this particular person, this might occur 1-3 times over the course of a bachelors degree program.
Other examples might be a student who, after 2 years of coursework, decides to do an overseas internship, research, or volunteer work for 3-6 months, before returning and continuing coursework; or a full-time student who, in the wake of an unfortunate family tragedy, needed to leave school for a period of time in order to take on the role of primary care provider for younger brothers, sisters, or children.
I think the majority of high school graduates have a natural gap between high school graduation and college. If the question was intended for this, it likely wouldn't exist, because it would apply to almost everyone. All this to say, if you check the box and your justification doesn't really apply, it'll probably simply be disregarded.