The flashback is a fantastic tool. With it, you can reveal the reasons behind a character’s motivation or psychology, a secret from their past, or the details of some previous even that help illuminate the “why” and “how” of the main plot. Flashback can be a double-edged sword, however. They interrupt your main storyline, bringing to a halt whatever momentum you had going.
Is the tradeoff worth it? It certainly can be, but keep in mind the following rule: The length of your flashback should be relative to the importance of the information revealed. For example, if your protagonist fondly remembers swimming in the local creek as a child, keep it short—a few lines of a paragraph at most. If, however, she witnessed a friend drown in that very same creek, and the details of their relationship and the event itself are critical to understanding her actions in the present, the flashback can be much longer—even multiple chapters.