The rotate modifier will rotate the children of the modifier. It will rotate all the children around the Z-Axis at the origin.
Start by adding a cube.
cube()

render the cube ( press f5 )
add the rotate transformation and render the model ( press f5 )
rotate(10) cube();

The cube has rotated in the clockwise direction by 10 degrees.
If you need to rotate the cube about its center, first center the cube then rotate.
rotate(10) cube( center = true );

You can rotate around any axis by providing a vector. Add
rotate([30,0,0]) cube();
