SEO (Search Engine Optimization) is the practice of improving search engine rankings to increase traffic to your website. Optimizing your site can be confusing and complicated. If you are in the process of hiring a web designer, ask them about their experience with SEO. There are many consultants out there who can get you started. But be wary of those firms that guarantee rankings. I recommend at least obtaining some familiarity with the concepts so you know what questions to ask.
If you are interested in learning the details yourself, below is an excellent resource.