A pre-existing condition is a health issue you had before enrolling in your health insurance plan.
With some health insurance plans enacted before the Affordable Care Act (ACA) went into effect in January 2014, an insurer could deny coverage (i.e., deny to pay a claim) for a health condition that pre-dated your policy. Or the insurer could charge you more for coverage than a person without a pre-existing condition.
After the ACA became effective, policies that were ACA-compliant, could no longer deny coverage for a health condition that pre-dates your coverage. ACA-compliant policies must cover treatment for pre-existing health conditions from the first date of coverage, with no waiting periods.