From the Super Mario Wiki
Flippers are bars that, once passed through, prevent access to the area before. They are commonly referred to as "pinball flippers" due to their resemblance to flippers from a pinball game. Yoshi can open them temporarily if he jumps on them.
Debuting in Super Mario World 2: Yoshi's Island, the flippers showed up again in Super Mario Advance 4: Super Mario Bros. 3 during the stages Slidin' the Slopes, Doors o' Plenty, and Swinging Bars of Doom.
A similar type of Flippers appears in New Super Mario Bros. in World 2-3 only. These types are wooden flippers with red arrows on them indicating which way it opens to, and are usually found individually rather than in pairs. Flippers exist unused in New Super Mario Bros. Wii; however, they reappear in New Super Mario Bros. 2 in a few levels.