A stratigraphic column is a drawing done by a geologist that describes what rocks in a cliff wall or underground look like. This allows other geologists, who have never seen these rocks in person, to understand the geology of that area, and perhaps even decide whether or not there is the potential for oil or natural gas to exist in these rocks.
Below is an example of how a geologist would create a stratigraphic column from rocks of the Grand Canyon.
On the left is a picture of the canyon. The center picture shows us how these same rocks have been divided by geologists in the past, based mostly on the differences betwen rock types as well as fossils observed within the rocks (different fossils tell us that rocks may be of different age, or were formed under different conditions, or both). The diagram on the right shows us a simplified stratigraphic column of this section of the Grand Canyon.
There are several key pieces of information contained in this stratigraphic column. First off, it shows us the major rock units for this section of the Grand Canyon. It also illustrates what types of rocks these units are composed of in two ways. First, by the name of the unit itself ("Coconino sandstone"), and also by the symbol used (the dots tell a geologist that the rock type is a sandstone). Lastly, it shows the relative thickness of the rock units. In this case, we see that the Toroweap formation is not as thick as either the Kaibab limestone or the Coconino sandstone. Don't be fooled by the fact that, in the scenic photo shown, the Toroweap formation appears to be much thicker than the Kaibab limestone. This is because the surface of the Kaibab limestone here has been weathered and eroded away. In places where the limestone has not been eroded, it is actually thicker than the Toroweap formation.