Question

what is compressibility in simple answer

Answer 1

Compressibility is the fractional change in volume per unit increase in pressure. 

Answer 2

it is the measure of the relative volume change of a fluid or solid as a response to a pressure (or mean stress) change.

{\displaystyle \beta =-{\frac {1}{V}}{\frac {\partial V}{\partial p}}}

where V is volume and p is pressure.