I am using Sharepoint 2013 Foundation.

After input of string to search field and enter key pressed, search results are displayed on search result page. May I know how Sharepoint determines the sequence in which these search result entries are displayed, i.e. which search result is displayed first and which is displayed second.......

Can I customize this sequence and how?

Thanks a lot for advice!

Foundation does not let you do much in the way of search customization. As far as how relevance works, take a look at this article:

http://www.sharepoint13blog.com/sharepoint-2010-search-ranking-model-and-results-relevance/