As you slip, the air streams around the fuselage above and below it.
In a high-wing aircraft, this will mean that the air that went around the bottom of the fuselage will stream up against the leeward wing, increasing the AoA of that wing and increasing the lift, thus rolling the aircraft away from the slip (which usually means towards level flight).
This is the reason for high-wing aircraft being inherently laterally stable to a larger degree than low-wing aircraft, where the air will be impacting the wing after having gone over the top of the fuselage, decreasing the AoA of the leeward wing.
If you have dihedral on the wings, a slipping turn will increase the angle of attack of the windward wing, as the air coming in from the side has a component perpendicular to the wing going up. That is why dihedral provides increased lateral stability. The opposite is of course the case for a wing with anhedral.
Cheers,
Fred