A flashback is a scene you use in your current narrative to show something that happened in the past. The two key differentiators are: 1) it must be a scene (as opposed to narration about an event), and 2) it’s past news.
Flashbacks are great for building three-dimensional characters because readers gains insight on how a character’s thoughts, feelings, and morals were formed by important events. They’re also useful for dropping hints about what happened to lead your main character to the current point in time. They help your readers understand and care deeply about your characters and what happens to them.