This just means putting new words, images, and pages on your website. This is one of the most important factors that Search Engines use to rank a site for search terms that are aligned with the subject matter of the content in question. Be warned; you cannot cheat the system! Your content must be of good quality and written with users in mind. Forget putting keywords in white text on a white background on your page, search engines will punish sites that do this; see Blackhat SEO Techniques.