“Prodromal” as an adjective refers to the early signs and symptoms of a disease before the “full” clinical signs and symptoms occur. It’s most often used to describe the signs and symptoms themselves or the phase of the illness. Examples include nonspecific symptoms such as mild fever, a headache, a mild sore throat, fatigue, or just generally not feeling great.
During the prodromal phase of an illness, it is often possible to detect the pathogen and test positive for an infectious disease, but sometimes a test will return a false negative, depending on the pathogen. For most pathogens, a person is infectious (contagious) during the prodromal phase even if their symptoms are very mild or barely noticeable.
The prodromal phase occurs after the incubation period but before the full on illness hits. It most often only lasts a day or two, but for some infectious diseases it can last several days or longer. For some non-infectious diseases, particularly psychiatric conditions such as schizophrenia, the prodromal phase can last weeks or years.