Bottom line
The IRR is the pool of Reserve Marines who are not drilling but can still be recalled to active duty. If you are in it, your main job is to stay reachable and stay screened: keep your contact and personal data current with the IRR's managing office, and respond to required training or screening. Fall out of contact, fail to participate when required, or ignore lawful orders, and you can be ordered to active duty to fix it, processed as an unsatisfactory participant, or separated.
What it means for you
You are in the IRR if you previously served on active duty (AC) or in the SELRES and are now non-drilling, whether or not you still have time left on your Military Service Obligation (MSO). You generally do not drill, but two things matter: trained Marines (those with an assigned primary MOS) can be called to active duty immediately in a national emergency, and everyone in the IRR has to keep their information current and submit to periodic screening so the Marine Corps knows you are mobilization-ready. The simplest way to protect yourself is to update your address, dependents, employment, and medical changes in Marine Online (MOL) whenever something changes.