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Pressure is force per unit area. There are two different ways to measure pressure: relative and absolute.

Relative pressure is the simpler of the two to measure. Take a surface of known area and attach a force gauge to it. Any difference in pressure from one side of the surface to other will give a net force directly proportional to the pressure difference, which registers on the gauge. For example, when we measure blood pressure, we are measuring it relative to the surrounding air. Many different gauges exist - springs, deflection of the sensor surface, various electrical devices, and even simply a vertical tube of water or another liquid: a pressure difference pushes the liquid upwards against gravity.

To measure absolute pressure requires something that produces no pressure on the surface of the gauge. This requires constructing a vacuum chamber behind the sensor surface. A perfect vacuum is impossible to manufacture, so the accuracy of absolute pressure measurements relatively poor.

For many applications - such as determining which way things will move - relative pressure is more important than absolute.

Pressure Relative To Where?

When measuring relative pressure, it is very important to understand exactly which two locations are being compared. As an illustration, consider the following thought experiment:

We drop a chicken egg into the ocean, attached to a dense weight to make sure that it sinks.

In question 3 above, the relevant difference was the difference between the inside and outside of our egg. In the ocean itself, pressure changes are due only to how much water is above you - so pressure is determined by depth (as long as we can treat the density of water as a constant). But the egg started inside a chicken on the surface and so is at lower pressure inside. As it sinks, the relative pressure grows.

Eventually, if relative pressures become too high, things break. It happens that eggshells are quite strong - in our thought experiment, the egg would sink tens of meters before it imploded. We normally break eggs and other things by concentrating the same amount of force into a smaller area; so that the pressure becomes greater than the strength of the material.