Google's Matt Cuts talks a little on this and gives excellent advice:
Visitors to your website can read your text straight away, however it's different for the search engines.
Search engines have to use programs called search engine spiders! for they can't read your website like a human eye is fit to do. They get there information from the page source code, through mathematical algorithms.
If you'd like to see what i mean look to the top bar of your computer, then look for where it says file, edit, format, view, help. click on where it says view, it will open a little window below, scroll down to you see page source and click it.