Pop-up Gate

From the Super Mario Wiki
A Pop-up Gate blocks a Snifit's bullets in Room 4-4

Pop-up Gates are objects in Mario vs. Donkey Kong: Minis March Again!. They are thin metallic walls that have a stop sign at the top, and extend from tiles with green upward arrows whenever the player taps them on the touchscreen. Pop-up Gates can be used to change the direction of minis or, in some cases, block bullets shot by Snifits. After activation, they stay up for five seconds before they retract back into their tile.

Pop-up Gates can be first encountered in Room 2-SP, though they are properly introduced through a tutorial in Magnet Mania.