Let us say we have one glass of water and after that when we add one or two spoon of salt then we notice that salt dissolves in it but when we measure the water level we found that there is no difference between initial reading and final reading. So what's happening over there ? Why water level doesn't changed when salt is added ?
This is an example of electrostriction - see, for example, this paper for a full explanation.
In short, the solvent molecules become more ordered in the vicinity of dissolved (charged) ions and less ordered as we go further away from the dissolved (charged) ions. The increase of solvent density near the ions is offset by the decrease of bulk density in the solvent as distance from the ions increases. This results in an overall net reduction in the observed volume of the solution.
From the Discussion section of the manuscript:
The interaction of the electrostatic field of an ion with water tends to align the dipolar water molecules in the direction of the field. In this way the field tends to disrupt hydrogen bonded structures in liquid water, and to compress the water molecules surrounding an ion. These electrostatic effect give rise to a shrinkage of the water.
Actually, sodium chloride added to water will decrease the volume of the solution, up to around 2% for a saturated solution. Even when fully saturated, that's not a big change, so you may not have been able to observe it without something with a narrow neck like a volumetric flask. If you start with a large volume and carefully mark the level, you should be able to see it.
Todd's answer explains why this happens with water, but it should also be noted that deviations from the ideal can happen in both directions (increasing or decreasing the volume) and changes in volume also occur when mixing two miscible liquids together, sometimes in a rather complex way. A good example is a binary mixture of acetone and chloroform (there's a good figure for this in the Engel and Reid physical chemistry textbook, but I can't find a decent one online). Starting from a pure acetone mixture, as we add chloroform, the volume is more than the sum of the two component volumes, but once we get above 20% chloroform or so, the volume drops below the sum of the two pure component volumes. Why this happens is a bit complicated, but it has to do with different competing factors affecting how compact the local structure around the molecules is.
When sodium chloride dissolves in water to make a saturated solution there is a 2.5 per cent reduction in volume. One would never notice that in a beaker. Even in an ordinary flask it would be barely perceptible. However, if a volumetric flask is available from the chemistry laboratory, the volume change will be noticeable in the narrow neck. It is essential to remove all air bubbles from the salt that is to be dissolved. Therefore, it must be thoroughly wetted at the start of the demonstration.
The solubility of salt does not change much with temperature, so there is little profit in using hot water.
The salt should be in small crystals and not in rocks or very fine powder.