Both Nikon P610 and Nikon L840 have 16.0 MP resolution sensors but Nikon P610's sensor is (1/2. ) and Nikon L840's sensor is 1/2.3' (6.17 x 4.55 mm ).
Since Nikon L840'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 Nikon P610.
Below you can see the P610 and L840 sensor size comparison.
As seen above, Nikon L840 has a infx Larger sensor area than Nikon P610. 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.