From the Super Mario Wiki, the Mario encyclopedia
A Control Block is a type of block found in Mario & Luigi: Partners in Time. It is a flat black and yellow block with a white arrow that cycles through the four cardinal directions. They are usually found inside tree stumps and other areas only accessible to Baby Mario and Baby Luigi. Control Blocks are always found near the platforms they control, which in turn are always found over beds of spikes.
The baby Mario Bros. must use the Control Block so that Mario and Luigi can cross the spike bed. When struck, the Control Block moves the platform in the direction the arrow was indicating when it was hit. Striking the block again causes it to stop, at which point another direction can be chosen. Careful manipulation of the Control Block and its platform allows the Mario Bros. to cross the spikes.
There is usually a Pipe Block on the other side of the spikes so that the older brothers can be reunited with the babies once across.