Incubation period is a term used in medicine.
When someone is infected with a pathogen (a disease-causing agent, like a virus), there is a time lag between when the person is infected and when the person has the symptoms of the disease (e.g. fever). This lag time is called the incubation period.
If someone is infected with the incurable HIV virus by having unprotected sex, then the virus will spend the next few months replicating in the host's body. For HIV, this incubation period can be many months, during which the victim will have no symptoms of infection (but can still infect other people). After the incubation period, the disease symptoms appear (which in the case of HIV, AIDS will occur and cause death).
For most infections, the body's immune system will not "gear up" or "kick in" until late in the incubation period.