what does Loss Tangent means in capacitor?
Generally we don't worry about the angle per se, but the fact that it represents loss. There are two situations where loss is undesired. 

In power applications, where loss results in significant heating.
In oscillator and filter applications, where loss results in lower Q.

In both cases, lower loss tangent is better.

There are some situations where ESR is a better expression of performance than loss tangent, although they are related by the capacitance. In power supply decoupling applications, it's a measure of the irreducible ripple that Farads alone cannot shunt. Here smaller ESR is better.