A flashback is 'a device in the narrative of a motion picture, novel, etc., by which an event or scene taking place before the present time in the narrative is inserted into the chronological structure of the work.' The flashback is a plot device used in many forms of media that should be used cautiously and with care.
Flashbacks can break the flow of a story. An author can be progressing along, have their readers invested in the current scene, and then break off into a flashback. This can pull a reader out of the story. That is something you want to avoid at all costs because a reader who loses the thread of the story will put it down and not pick it up again. Kristen talks about the problems with flashbacks here.
The thing to keep in mind with flashbacks is while there are suggestions, there are no hard and fast guidelines. There are also devices that may appear to be flashbacks that aren't. There are many examples of those, which you can see here.
A story full of flashbacks causes problems for the reader as well. Generally, if an author has to put a large number of them into their manuscript, then there is a very good chance the author isn't telling the right story. If the events are so important that the flow has to be disrupted more than once or twice the story should be overhauled to make a more continuous storyline. There is some discussion of that here.
Flashbacks should be kept to a minimum or eliminate them completely. Find a method for conveying the information in a more linear fashion. If it is backstory, look for a way to insert it where it doesn't disrupt the progression of the story.