There are distinctions between (a) providing backstory in a flashback scene; (b) providing backstory as an integral part of a character’s current thoughts; and (c) providing information about a character’s backstory as a narrative summary. The first two are preferable as they keep the story’s live action, as it were, moving forward.
With respect to (a), even if a scene is in flashback, it is still a scene, and if it is engaging as any scene should be (it contains tension, it has a purpose within the story), it won’t take the reader out of your story.
With respect to (b), backstory that comes up in a character’s thoughts just blends smoothly into the current action, and is often the best and most seamless way to fill in backstory. For an off-the-cuff example, “Jane hadn’t seen a linoleum floor like that one since the trailer she and Walter lived in when they first went to Los Alamos during the war. Its dull brown tiles reminded her of the late nights he spent at the lab in those days, and the long silences when he finally came home.” <— That sentence is a thought triggered by Jane seeing something in her present scene, and allows you to infer some things about Jane’s backstory: she’s married to a man named Walter who worked on the Manhattan Project, and she went there with him; they lived in a trailer, and their marriage might have been a little strained during that time.
It is (c), when you take the reader out of the story to provide a distant narrative summary of past events, that runs the risk of pulling your readers out of the story and dumping information on them, rather than showing them scenes that engage and illuminate. “During the war, Jane and her husband Walter lived in a trailer in Los Alamos, while Walter worked long hours on the Manhattan Project. Walter could not talk to Jane about his work, and so he didn’t talk to her much at all. The trailer had a linoleum floor.”
Can you see why (b) might be more compelling than (c)? (b) is immediate, in Jane’s POV, and blends into whatever Jane is doing right now in the scene. It also builds reader curiosity by saying things a little indirectly. (c) is distant, not connected to the current actions of either Jane or Walter; it’s just dumped there. By working bits of (b) in and throughout your present action, rather than interrupting the action to dump paragraphs full of (c), you build reader curiosity and maintain their interest, all while keeping them in the middle of your story.
There are times and places for all kinds of exposition, of course, but if you are getting feedback about having too much infodumping, or not working your backstory smoothly into your current story, the distinction I’ve tried to explain here is worth thinking about. One book that discusses this very helpfully is Sol Stein’s Stein on Writing.