Everything you see on your browser screen is essentially a page on a specific site. Every page will likely have a title or heading followed by the body text and other content. There is also information on each page that you don't see. It resides in the background in the language used to write the page. What you see and don't see on your screen, is important when searching.
Some search engines look for titles and headings.
Others look for specific text or words in a document, while others look for fields in the HTML code in the background. That is why different search engines require that you enter your queries differently. Some suggest using 'Boolean Operators' to refine your search. Other engines look for words in close 'proximity' to one another. Some will look for 'phrases'.