Search engines might as well be called keyword search engines because that is what they really are. They help people find websites related to the keywords or keyphrases they search on. So, if you want to receive free traffic from search engines, your best bet is to convince the search engines, especially Google, that your website matches the keywords used by your customers. Not only that, but you have to convince the search engines that your website is MORE related to those keywords than your competitors' websites.
Furthermore, you have to make sure that those keywords are actually what your customers. You are not looking for just any traffic, you are looking for targeted traffic: customers that buy. Using the wrong keywords will disappoint the users searching on those keywords, and confuse the real customers who would be interested in paying for your services and products.
Therefore, quote an effort needs to be spent on selecting the right keywords for your website. You need to: