Each search engine has its own way of doing this, which is called its search algorithm. Some pull out words that are believed to be significant and ignore words believed to be insignificant. Some index every word on a page, some only part of the page. Sometimes words that are mentioned towards the top of a document and words that are repeated several times throughout the document are more likely to be considered important. For the most part, search engines do not understand synonyms.
Some navigators are more useful for one subject than another.
Most yield better results if one is able to do fairly precise searching on clearly defined concepts. Because the Internet is always growing and because these search engines search in different ways and search different parts of the Internet, doing the same search using different search engines will often give you wildly differing results. Eventually one learns with practice which search engines work best for individual searching styles and topics. But the learning is a continuing process. The most important thing to remember is not to get stuck just using one method of searching or one search engine, especially if the results are poor.