the "flats" usually discussed here are the Susquehanna Flats.
it is located at the top of the Bay on the western shore where the Susquehanna River meets the Bay & the Northeast River.
it is called the flats because, even though it cover a large area it is VERY shallow across most of the area.
it is an excellent place to catch large rock as they come up to the head of the Bay following the shad, etc. and to spawn, however, they do not spawn on the Flats
as the water warms and the fish move in the boards will be chuck full of reports from up there.
later in the season you can catch a large variety of species from LM bass to shad, catfish, white perch, etc.
the preferred tactic for targeting rock in the spring is LTJ (light tackle jigging) where you toss lead head jigs with soft plastics on them in the pockets of the shallows.
trolling is generally not used in this area for thr bigger rock.
the trolling / trophy season will begin in the Bay proper as the stripers come up from the ocean to spawn.
i don't have the opening day date right now but i'm sure it willl be posted.