It means exposure to the event where you could have been likely infected by the virus

Exposure in this case means risks or unprotected. How many times you put yourself at risk of contracting the virus through conventional methods such as unprotected sex, IV drug usage (shared needles), etc. You were exposed to HIV when you had unprotected sex. It will take 4 weeks to know if you are truly negative.

"Exposure" is the time you were exposed to the virus. This is not only for HIV, but any virus. For example, I'm teaching in class, and a child coughs on me. This was my exposure to the cold virus. Do you understand textbook English recordings but struggle to understand what you hear or read in real English situations? For more help with real life language problems, I recommend signing up for a language program. I offer some you might be interested in.