Reverse Engineering is basically taking a product that already exists and copying the design or analysing how it works.
The reason why people reverse engineer things is to figure out how a product was made to help construct a new device but without directly copying anything from the original.
To copy a part exactly could very likely infringe on some existing patents the design may have, but changing if totally or improving on the design is usually the objective.
Generally speaking most designs out there are very similar, lets think about a printer for instance. The printer feeds paper in one side through rollers, ink is printed on the paper with a mechanism that holds ink cartridges that moves from side to side and then the paper is expelled out the other end. The principle and working method of the printer is basically the same for all printers. The way that the individual companies varied the design and put their own twist of certain design areas is how they get around patents if they are any. It is very dangerous to directly copy someone else’s design but to look at how the design works and modify it to work another way is reverse engineering. Reverse Engineering will help you get unstuck with a design issue you may have been having giving you ideas to think about.