Magnet sliding down an aluminium incline

A magnet that slides down on a metal slope (or inclined plane) will experience a braking effect due to the magnetic fields of the currents induced in the surface of the slope. This is a distinct phenomenon from the usual mechanical friction. This only occurs because the slope is a conducting material (aluminium/aluminum in this case). Also note that aluminium is not ferromagnetic (in other words, it is not attracted by magnets)!
The motion of the magnet down the slope under the effect of gravity is the action that is responsible for the creation of induced currents in the surface of the slope. These currents have magnetic fields associated with them. The magnetic fields have such an orientation that they hinder the action that created those induced currents. This is known in physics as Lenz's law (or Lenz's rule). Lenz's law is an expression of the law of conservation of energy.