By chance,
Are you changing careers?
Have you gone back to school?
Have you taken care of family?

Volunteering is significant and can let them know that too, even to put it on your resume to list as "Other or Additional Experience" with company/organization name, location(s), and the date(s), to show you have not been idle during this time. If asked, let them know something along the lines that you 'wanted to take some time to help out in the community, while you reprioritized (or changed) your career plans/goals.'