MLCC capacitor - AC voltage performance
Can any one help me to understand the meaning of such curve.

It's indicating a safe operating area for current. As frequency rises and VAC is held constant, the current rises so, using the solid black line at 100 kHz (for the 100 nF capacitor), there is an AC RMS voltage limit of about 220 volts and, I expect that at 1000 kHz, the applied voltage limit is one-tenth of this at 20 volts AC (RMS).
In other words, this graph is ensuring that you don't put too much current through the capacitor.