From the Super Mario Wiki
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A One-Way Wall is an object found in Super Mario Maker, Super Mario Maker for Nintendo 3DS, and Super Mario Maker 2. Similar to the Flippers from the Yoshi's Island games, once Mario passes through the gate, he cannot go back through it from the other side. A One-Way Wall is the size of four regular blocks, and the arrow imprinted on it indicates the direction in which Mario can enter.
One-Way Walls can also be used in conjunction with Warp Pipes to recreate one-way pipes found in several Super Mario games. Additionally, when pointed upwards, One-Way Walls can be used similar to semi-solid platforms.