The dimensions, the shape, and the internal structure of an object determine the way it can vibrate. A plate does not sound the same way when it is cracked; a glass does not sound the same way depending on its shape or how high it is filled.
The ensemble of "vibration modes" of an object is specific and it is something that characterizes it. The associated frequencies reveal its structure.
Detecting and measuring these vibrations give us information on the object, in particular on its interior (inaccessible to direct observation in the case of the stars) and on what causes the vibrations.
The stellar sismology enables us to study the interior of the stars.
The stars have permanent periodic movements, observed for the first time in the Sun about twenty years ago. These vibrations are, in general, very faint and very difficult to observe in distant stars.
However, analyzed with sismology techniques, they allow us to "observe" the interior of the stars, often very different from the Sun.
By observing these oscillations in stars with different masses, ages and origins, we will be able to determine the history of their evolution. That's what CoRoT will reveal for the first time.