An ADHD screening is a standardised questionnaire that asks you to rate the extent to which you experience attention difficulties, impulsivity and hyperactivity. Well-known examples include the CAARS-S:S (Conners' Adult ADHD Rating Scales) and the ASRS-v1.1 (WHO Adult ADHD Self-Report Scale).
Such a questionnaire provides an indication — not a diagnosis. A high score means your symptom pattern aligns with ADHD symptoms; a low score does not rule out ADHD. Both outcomes require interpretation by a clinical professional who knows your full medical history.
What online tests can do: they help you articulate what you are experiencing, and they can start the conversation with your GP or treating clinician. That is valuable — as long as you do not treat the result as definitive.