This is basically how the search engine spider crawl through your website.
Basically your website get indexed and ranked when the search engine spiders crawl trough your website. Below is how to google search engine works.
First, Google spider crawls out from the base computer system in its search engine. After crawling out, the spider will go out and check for website or web-pages. When spider found a web page, it will start to read through the web-pages. It will start off with the title tag (between open <title> and close <title> tags). Here, it will read through your keywords or phrases, description tags and meta tags.
After that, the spider will start to read the content of your page. It will take note of keywords or phrases that appeared in your content which has been specified in keywords and description meta tags. It will also take note on the heading you use as larger heading get higher priority. Also, it will also take note on your picture alt tag description and this is why you are recommended to put alt tag for all your pictures.
Now comes the important process related to the ranking of your website. When the spider found a link, it will take note of the links title and follow the links. You get the traffic benefit if the web page is linking to your website. The spider reaching your website will then repeat the process again and start off by reading through the head to body tags of your website.