Adept means very good at something — but not at just anything. It is for skills that need practice, care, and a bit of cleverness: playing an instrument, handling difficult people, cooking, fixing machines, arguing a case. You would not normally say someone is adept at sleeping or adept at eating. The word points at a skill that was learned, and learned well.

The usual pattern is 'adept at' + doing something: 'She is adept at calming angry customers.' You will also see 'adept in', especially in more formal or written English: 'adept in the use of these tools.' Both are fine; 'at' is far more common in speech.

It can also be a noun — an adept is a person who is highly skilled, and there the word carries an old flavour: for centuries it meant someone who had mastered a secret art, like alchemy. That colour still clings to the noun a little, so 'an adept' sounds bookish, almost mysterious. The adjective is the one you will actually use.

Though actually — adept sits a step above 'good at' and roughly level with 'skilled', but it is a written, educated-sounding word. In everyday Taiwanese classroom English, students hear 'good at' and 'really good at' far more. Save adept for writing, or for when you want to sound precise. And notice the quiet compliment inside it: calling someone adept says they made it look easy, which is a nicer thing to say than 'strong' or 'able'.