In the description of a container with a hole as a blackbody, in Peter Atkins | Julio de Paula - Physical Chemistry, It's given that
"the container with pin hole emits radiation that can be thought of as oscillations of the electromagnetic field simulated by oscillations of electrical charges in the material of the wall of the container."
and it is also stated that
" The radiation which enters through a pin hole is reflected many times within the container and comes to thermal equilibrium with the wall."
My understanding is:
The temperature of the container proportional to the frequency at which the atoms of the container vibrate. "Oscillations of electrical charges" in the first statement refers to the oscillation of charges due to the oscillation of atoms in which they are present. "Thermal equilibrium" in the 2nd statement means that the frequency of the radiation becomes equal to the frequency of the oscillating atoms (which depends on the temperature) after some time.