A retrospective study looks back at information that already exists, such as medical records or people's memories, to look for links between past events and current outcomes.
They are quicker and cheaper than prospective studies because the information already exists. The trade-off is quality. Records were not written with the study in mind, and memories change over time.
Most adult ADHD and autism assessments have a retrospective part. The clinician has to establish that traits were present in childhood, sometimes decades later. That's why school reports, baby books, old photos and someone who knew you as a child are so useful.
If you remember little of your childhood, say so. That is common, and it is information the clinician can work with.