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Overburden pressure is the vertical stress imposed by the overlying formation at a reference point below the surface. In other words it is the hydrostatic pressure exerted by all the material above a reference point. The overlying layers can include rock columns and bodies of water. Why is it important? Because we can use it to determine the fracture pressure of the rock. The figure below shows a visual representation of what overburden stress is:
From the figure above, it is clear that the overburden stress at a depth 
, the datum point, is the sum of the pressure caused by the fluid column and the pressure caused by the rock (fluid-grain mixture). It is important to note that we use bulk density when calculating the pressure caused by the rock column, NOT the grain density. If your still having trouble with the bulk density concept, think of the rock in the subsurface like a wet sponge. The sponge material are the grains, and the voids in the sponge are filled with water. You can account for both of these by using bulk density.
Because the overburden stress is the sum of all the material above a reference point, the mathematical expression to calculate the overburden stress is represented by the following expression:
(1) 
where:
= the overburden stress
= density of the fluid column as a function of depth
= gravitational constant
= the bulk density as a function of depth
= the depth
The first term on the right hand side of equation (1) represents the pressure caused by the water column. The second term on the right hand side of equation (1) represents the pressure caused by the rock column. A lot of times, petroleum engineers assume average density values for the fluid column density and the bulk density. Equation (1) can then be simplified. In oilfield units the overburden stress can be determined by the following expression with the simplifying assumptions:
(2) 
where the densitys have units of 
and the overburden stress ends up with units of 
. In real world applications, the bulk density varies with depth because the porosity changes with depth. i.e. the deeper you go, porosity tends to decrease because the rock is compressed more due to supporting more material above it. You can definitely read up more about how to account for this, but a lot of times it’s easier to assume a value of 1.0 
as the pressure gradient for the rock column. This is good ballpark number if you want to quickly do a hand calculation or do not have the data to calculate an average bulk density value. So a general rule of thumb is the following:
Assume a rock column load of 1.0 
if you lack information
In upcoming discussions, we will tie the pore pressure and oveburden stress concepts together to determine the effective stress on rocks in the suburface.