In case of pendulum clock,lets say one swing ticks one second..but what is the analogy in case of CAESIUM atomic clock? Is 9,192,631,770 ticks is equivalent to one tick in pendulum clock? And how we come to define one sec as this much number of oscillation? and how this process keeps on continuing?do we need to provide CAESIUM atoms again and again?
Every atom, including cesium-133, emits (or absorbs) electromagnetic waves (light or its generalization to invisible colors) when the electrons jump from one state in the atom to another.
The electromagnetic radiation is a periodic process in which the electric (and similarly magnetic) fields at a given point of space behave as $$ E = E_0 \cdot \cos (2\pi f t) $$ where $f$ is the frequency. The frequency $f$ is absolutely determined by the difference of energies of the atoms before and after the transition, $E=hf$.
The particular transition used to define one second is a tiny transition between the split ground state of cesium-133. All the shells of the electrons are filled except for the lonely valence electron that sits in the $6s$ shell. The relevant transition doesn't bring it to a higher $6p$ shell or something like that.
Instead, the $6s$ shell is split due to the very weak interaction of the electron's spin with the nucleus' spin. This splitting is known as the "hyperfine structure" and the corresponding energy (and frequency) is some 100,000 times smaller than the energy (frequency) needed to ionize the atom.
With lots of cesium-133 atoms, one may really observe the microwave electromagnetic radiation. The most accurate existing technical solution for the atomic clocks is currently based on the atomic fountain, an update of the Ramsey method to measure the frequency of the transitions.
The property of Caesium that makes it such a stable oscillator is the lone electron in its $6s$ orbital. All other electrons in the lower energy levels take a symmetrical electron configuration and leave the $6s$ as an "outsider".
The spin of the Caesium nucleus can cause a so-called hyperfine transition in that $6s$ electron which has a very specific energy associated with it. This hyperfine transition absorbs radiation of the exact frequency of 9,192,631,770 Hz. As explained on the HyperPhysics website, this transition has an incredibly low but highly accurate energy value.
The actual measuring technique is quite complex but it involves vaporizing caesium and then separating the atoms by spin state using a magnetic field. The lower energy spin atoms are then subjected to microwave radiation and once again separated according to spin. This time the atoms with a higher spin energy are ionized and detected by a mass spectrometer. Tweaking the frequency of the microwave radiation can allow researchers to find the resonant frequency when the number of particle detections reaches a maximum.