Maxwell's second equation is the differential form of Gauss's law of magnetism.
As magnetic, monopoles do not exist in magnets and the magnetic field lines form closed loops. There is no source of the magnetic field from which the lines will either only diverge or only converge. Hence the divergence of the magnetic field is zero.
$\overrightarrow{\nabla}. \overrightarrow{B}=0$

Derivation
According to Gauss's law of magnetism
$\oint_{S} \overrightarrow{B}. \overrightarrow{dS}=0 \qquad(1)$
Now apply the Gauss's divergence theorem
$\oint_{S} \overrightarrow{B}. \overrightarrow{dS}= \oint_{v} \overrightarrow{\nabla}.\overrightarrow{B}.dV \qquad (2)$
from equation $(1)$ equation $(2)$
$\oint_{v} (\overrightarrow{\nabla}.\overrightarrow{B}).dV =0$
$\overrightarrow{\nabla}.\overrightarrow{B} =0$
