Adherence to treatment means how closely day-to-day treatment matches the plan agreed with a clinician, such as taking medication at the agreed times or attending sessions.
Clinicians now say adherence where older texts said compliance. It describes what happens. It is not a judgement on whether someone is behaving well.
With ADHD, following a treatment plan uses the same skills ADHD affects: remembering a dose at the same time each day, reordering medication before it runs out and booking the review appointment. Missed doses are common for that reason. They are useful information for your clinician, not a personal failing.
Practical systems work better than relying on memory. A phone alarm. A pill organiser next to the kettle. A repeat order set up with the pharmacy. If side effects are the reason doses get skipped, say so at your review, because the plan can change.