Health maintenance organizations, or HMOs, are a type of health insurance plan. If your coverage is an HMO, you'll need to pick a primary care physician (or your insurer will pick one for you), and that person will serve as a "gatekeeper," meaning that you'll need to see your primary care physician for a referral before you can see a specialist.
This is in contrast to PPOs (preferred provider organizations), which allow members to see any doctor in the network (and in many cases, outside the network too, albeit with higher cost-sharing), without a referral. The trade-off is that HMO coverage is typically less expensive than comparable PPO coverage.
Out-of-network care generally isn't covered at all under an HMO, unless it's an emergency situation.
In the individual insurance market, HMOs have become much more common (and PPOs have become less common) since the bulk of the ACA's regulations took effect in 2014.
