Term boosting, in the context of search engines, is the ability to assign higher importance to specific words in a query.
In Google search, for example, advanced search options allow you to identify words that must be found in the results. Other search engines allow you to identify important words in your search terms by appending a character to them. More sophisticated options, such as assigning ranked importance, are available in some search engines.
NASA's Global Change Master Directory, for example, allows users to identify particularly significant words in search terms by affixing a caret (^) at the end of a word along with a numeral (higher than 1, which is the assumed boost rank of all terms) to indicate the relative importance.