When you talk about something in detail, you explain it thoroughly. You might describe your Disney World vacation to your coworkers in detail, for example, including every single ride, all your meals, and the fight you and your sister had.
Use the adverbial phrase in detail when something is explained or described fully, leaving out no fine points. A good clothing catalog describes items in detail, and a sharp photograph can show a person's features in detail. Newspaper articles explain current events in detail, including every particular that a reader might want to know. The simplest way to think about something done in detail is that it covers all the details.