Both Sony A7 III and Sony A6400 have 24.0 MP resolution sensors but Sony A7 III's sensor is Full frame (35.8 x 23.8 mm ) and Sony A6400's sensor is APS-C (23.5 x 15.6 mm ).
Since Sony A7 III'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 A6400.
Below you can see the A7 III and A6400 sensor size comparison.
As seen above, Sony A7 III has a 2.3x Larger sensor area than Sony A6400. 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.