Stories are usually not told entirely in chronological order. Most have at least one flashback, if not more. The flashback can take a story back and provide information that also moves a story forward.
For example, if we find out that a teen had been abused in some way by a parent, teacher or other adult, it can explain his current emotions, actions and motives.
Flashbacks do have to be handled carefully, however. A reader can get so caught up in the past that he/she does not want to return to the present. Or, a reader can get impatient with a flashback and wonder when it will end so that the story can move on. They must written so that they are seamless and enhance the story rather than slow it down.
Reread any flashbacks in your stories and see if you get too involved or too impatient. If you do, so will your readers.