Refer to figure.
As you can observe in the attached figure, in a LOW, air is converging at the surface (being sucked in at low level) and rising from the surface to higher altitude and, thereafter, diverging.
This lifting action cools the air adiabatically => if moisture is available, it may lead to condensation and formation of clouds.

Converging near the ground and general lifting within a LOW lead to formation of extensive stratiform cloud layers and may strengthen convective movements, which leads to the formation of CB clouds (if the air mass in unstable).

Note: Trough is an elongated region of relatively low atmospheric pressure.