“Record of” means that the person has a history of, or has been misclassified as having, a mental or physical impairment that substantially limits one or more major life activities, even though the person does not currently have a disability. For example, a person who undergoes treatment for cancer then returns to work. Although the cancer may not be in remission, they have a record of having had it.
For additional information, take a look at the following resources:
FAQ: What is the definition of disability under the ADA? [2]