Both Leica TL2 and Leica SL have 24.0 MP resolution sensors but Leica TL2's sensor is APS-C (23.6 x 15.7 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 Leica TL2.
One other difference between these two cameras that is worth mentioning is that Leica SL's sensor doesn't have an 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 moiré occurring in certain scenes.
Below you can see the and SL sensor size comparison.
As seen above, Leica SL has a 2.3x Larger sensor area than Leica TL2. 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.