Prior Art is a generic term referring to'stuff people have already done.' It might include intellectual property, such as patents and copyrights, or it just might come from observing artifacts of various sources. For engineers, the patent literature is typically very important, not only for investigating what others have done, but also to find technologies you might have to license in order to incorporate into your designs.
Other sources of prior art can be simple searches of trade magazines, catalogs, and the like, even basic image web searches, to see what components or technologies are currently in use.
For more detailed information about patents, check out the full Patent libguide.