Take a moment to look around the room in which you're reading this book. At first, you may think that there really aren't that many things to observe: some furniture, light fixtures, perhaps some plants, artwork, even some other people, or pets. Maybe there is a window to gaze out of that opens up the outside world to observation.
Now, look again: for each thing that you see, there are a myriad of details to observe: its size, its color, its intended purpose, the components from which it's assembled (the legs on a table, the lightbulbs in a lamp), etc. In addition, each one of these components in turn has details associated with it: the type of material used to make the legs of the table (wood or metal), the wattage of the lightbulbs, etc. Now, factor in your other senses: the sound of someone snoring (we hope not while reading this book!), the smell of popcorn coming from the microwave oven down the hall, and so forth. Finally, think about all of the unseen details of these objects—who they were manufactured by, or what their chemical, molecular, or genetic composition is.
It's clear that the amount of information to be processed by our brains is truly overwhelming! For the vast majority of people, this doesn't pose a problem, however, because we're innately skilled at abstraction: a process that involves recognizing and focusing on the important characteristics of a situation or object, and filtering out or ignoring all of the unessential details.
One familiar example of an abstraction is a road map. As an abstraction, a road map represents those features of a given geographic area relevant to someone trying to navigate with the map, perhaps by car: major roads and places of interest, obstacles such as major bodies of water, etc. Of necessity, a road map can't include every building, tree, street sign, billboard, traffic light, fast food restaurant, etc. that physically exists in the real world. If it did, then it would be so cluttered as to be virtually unusable; none of the important features would stand out.
Compare a road map with a topographical map, a climatological map, and a population density map of the same region: each abstracts out different features of the real world—namely, those relevant to the intended user of the map in question.
As another example, consider a landscape. An artist may look at the landscape from the perspective of colors, textures, and shapes as a prospective subject for a painting. A homebuilder may look at the same landscape from the perspective of where the best building site may be on the property, assessing how many trees will need to be cleared to make way for a construction project. An ecologist may closely study the individual species of trees and other plant/animal life for their biodiversity, with an eye toward preserving and protecting them; a child may simply be looking at all of the trees in search of the best site for a tree house! Some elements are common to all of these four observers' abstractions of the landscape—the types, sizes, and locations of trees, for example—while others aren't relevant to all of the abstractions.