What is Hydrostatic Pressure?

While drilling a well, drilling fluid (mud) is pumped through drill string, it comes out through bit nozzles, enters the annulus, travels up the annulus and finally reaches the surface and comes back to mud pit through flow line. Figure below is illustrative of what i have stated.

Thus in the process of drilling the annulus is always kept full of mud.

Primary Well Control

During normal drilling operation, we always keep the wellbore full of drilling fluid (mud). The drilling fluid column in the wellbore exerts a pressure at the bottom of wellbore, called hydrostatic pressure.  Hydrostatic pressure is the pressure acting on wellbore when the mud pumps are off and there is no circulation of fluid in wellbore. 

Also, the compressed formation fluid contained in the formation has a trapped pressure called formation pressure. If the hydrostatic pressure of drilling fluid column is greater than or equal to the formation pressure, the formation fluid cannot enter the wellbore and we say that the well is under control.If the hydrostatic pressure of drilling fluid column falls below the formation pressure, the formation fluid starts flowing into the wellbore. In this situation we say that the wellbore has taken a kick.