Background is the information gathered about a person before or during an assessment, such as developmental history, school reports, family history and previous diagnoses or treatment.
In research, background also means the section of a paper that explains what was already known before the study. On this site we mostly mean the clinical sense.
For ADHD and autism assessment, your background matters more than in many other assessments, because both conditions are present from childhood. A clinician needs to know how you were as a child, not only how you are now. Old school reports help. So does a parent, sibling or long-time friend who can describe you as a child.
If you don't have school reports or anyone to ask, tell us. That happens often, especially for adults assessed later in life, and the clinician works with what is available.