A capsule is a medicine contained in a small shell, usually made of gelatine or a plant-based material, that dissolves in the stomach or gut after swallowing.
Several ADHD medicines come as capsules. Some long-acting capsules hold small beads that release the medicine at different times across the day.
Whether a capsule can be opened depends on the specific product. Some are designed to be opened and the contents mixed with a small amount of soft food or water, which helps people who find swallowing difficult. Others must be swallowed whole, because opening them changes how the medicine is released.
The patient information leaflet in the box says which applies. If it isn't clear, ask your pharmacist. Swallowing tablets is harder for a lot of adults than people often say, so it's worth raising at your medication review.