Professionalism in a basic sense means acting like a professional at what you do. Professionalism affects how others perceive you, whether they be your coworkers, managers or clients.
When not acting in a professional manner, you may lose credibility, opportunities for advancement or even your job.
How professionalism is demonstrated varies by industry and workplace. Some places value the way you look and act more than other things. Other places may place importance on how you talk or even what your hobbies and interests are.