In the hydrides of VI A group elements, the acidic strength gradually increases from top to bottom. This is due to

 $\mathrm{Ka}$ is called the dissociation constant.

$\mathrm{Ka}$ value can be used to determine the acid's strength: If Ka is high (and pKa is low), the acid is largely dissociated and therefore powerful. 

Therefore higher the $\mathrm{Ka}$ value, higher is the acidic strength and vice-versa.

Thus,  In the hydrides of VI A group elements, the acidic strength gradually increases from top to bottom. This is due to increase in their $\mathrm{Ka}$ values