This is a sketch of an enhanced oil recovery technique. After primary methods, man goes ahead and tries secondary, then further tries tertiary or enhanced oil recovery. But no matter how far we go, we still cannot squeeze out all the oil or gas from any reservoir rock. This is why it's important to understand the concept behind this term - residual oil saturation.
Try pouring a glass of water on a napkin, now try squeezing the napkin dry with your hands. No matter how hard you squeeze, you still cannot take out every molecule of water from the cloth. Unless you make use of a dryer or place the napkin under the sun.
Now, unfortunately we cannot dry our reservoir rock. So our methods are limited. The goal however, is not to squeeze out every drop since we can't, the goal is to recover or squeeze out as much as possible. When we understand how much of the hydrocarbons we cannot recover (residual saturation), we can now focus on what we can recover and try every economical method to help us do so. Now, we have a standard to measure how efficient our methods are.