The ultimate goal of any SEO strategy is to dominate the top of the search engines. Before this can even begin to happen, though — and way before on-site optimization and inbound links become a factor — your website needs indexing.
Indexing simply means that your content is recognized by the search engines and can be discovered via relevant search queries. In short: if organic search engine traffic is an important part of your long-term strategy, being regularly indexed by the likes of Google, etc, is essential.
Of course, for your website to be indexed, this requires a little visit from the infamous Googlebots. These robotic critters “crawl” your site, searching every nook and cranny in their hunt for new web pages. When new (or updated) content is detected, the URL is passed back to the search engines to be indexed.