One of Lightroom’s greatest strengths is the database (known as the catalog file) that records everything you do when working in Lightroom. When you import photos all the information contained in the metadata gets added to the catalog, such as exposure, ISO, time and date of capture, f-stop, camera model, and so on. In addition you probably add your own custom metadata, such as contact information, copyright, titles, keywords, ratings, labels, and so on.
Now that you have all that data entered into the catalog it only makes sense that you should be able to leverage it to make your work more efficient and productive. This is where smart collections come in. If you open the Library module and expand the Collections panel you will find a default collection set labeled Smart Collections.