Both Sony A6000 and Leica SL have 24.0 MP resolution sensors but Sony A6000's sensor is APS-C (23.5 x 15.6 mm ) and Leica SL's sensor is Full frame (36 x 24 mm ).
Since Leica SL's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Sony A6000.
Another difference between these two cameras is that Leica SL's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the A6000 and SL sensor size comparison.
As seen above, Leica SL has a 2.4x Larger sensor area than Sony A6000. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.