It is making sure Google can read the contents of your website. For example, when you stuff all your text in an image, Google’s automated process won’t be able to read it. However, if you annotate stuff with related information in the code (this needn’t be visible to your visitors), Google can use that. There’s a lot of similar facets to it (e.g. make sure all your pages use just one way to get there so Google doesn’t spread the page’s ranking across multiple links). It’s not that difficult, IMHO, just a lot of things you can pay attention to, and lots to read up on. Mostly, I just try to write valid HTML and accessible pages and on providing good content that Google will want to list high.