If you are buying your own individual plan:
Guaranteed issue is a requirement by your state government that a health plan must allow you to enroll regardless of your health, age, gender, or other factors that might predict your use of health services (such as a pre-existing condition).
You can learn about your state’s requirements from the website State Health Facts.
If you are buying health insurance for a small company:
Federal law (and all the states) requires that all health plans sold to companies with 2 to 50 employees be on a guaranteed issue basis. This means that small employers cannot be turned down by a health based on the health status of its employees.
If you are self-employed and have no employees, the federal law does not require that health plans sell you a health plan on guaranteed issue basis. However, some states do have more extensive guaranteed issue requirements. For example, some states define a small employer as having 1 to 50 employees, thus requiring health insurers to include sole proprietors in guaranteed issue plans.
The State Health Facts website outlines the requirements for each state: Small Group Health Insurance Market Guaranteed Issue