what is mean by "energy is quantized"?

[Admin]

It means that Energy can only change in discrete steps, not continuously.

Consider this scenario: You want to fill an empty bucket with water.
One option is to use a tap and the other option is to throw ice cubes into the bucket which would eventually melt. Suppose each ice cube is made of 100 ml of water. So, you can only fill up the bucket with 100 ml,200 ml,300 ml..etc of water. So the amount of water is "quantized". You can never fill the bucket with say,150.567 ml of water using this method. However, if you use a tap you can fill up the water up to any level. In this case the amount of water is "continuous".
Energy Quantization is of interest when studying electronic energy level in atoms and while studying Photons. In isolated atoms, the energy levels are discrete, which means that the electrons can either have energy E0,E1,E2 etc. but not anything in between these energies.
Photons are packets of electromagnetic energy. Their energy depends on the frequency of the EM wave. If light falls on an object, the object can only absorb an integer number of photons.

[Admin]

thats nice explanation