Two common terms you may here in the home insurance industry from time to time are endorsement and rider. Fortunately they are not hard to remember because essentially they are the same thing. These are written forms that are attached to your insurance policy. They are there to modify terms and policies or add/remove certain types of homeowners insurance to your existing plan.
For example, if you want to add hurricane coverage to your homeowners insurance, a form would be attached to your policy that adds this coverage along with the appropriate cost and terms and conditions. Simple.
Obviously, endorsements or riders are mainly used when you want to augment your insurance to cover something that you feel is necessary but was not part of your original policy. This way, you can alter your insurance at any time without having to wait until the current policy expires before adding something else you need.