When we enter search terms into a database, we're looking for items related to a concept that we have in mind. However, databases don't search for concepts, they search for the exact words that you've entered. If you're looking for studies that have taken place in the Rocky Mountains, you might use Rocky Mountains as a search term. However, if there are journal articles that describe studies as being conducted at Glacier National Park or White River National Forest, those articles would not appear in your search results unless the term Rocky Mountains was also used somewhere in the article's text.
If you want to search for an exact phrase of two or more words, use quotation marks. Searching for "Rocky Mountains" in quotation marks will get you results that only use those two words in that exact order. Searching for Rocky Mountains without quotation marks would pull in results with that phrase, but could also pull in an article that talks about rocky soil near mountains.
Be cautious about using this strategy if you'd be willing to accept instances where the words are not used in an exact phrase. For instance, "evolutionary mechanism" would not pull results that use the phrase mechanisms of evolution.
The Boolean Operators AND, OR, and NOT can be used to widen or narrow the scope of your results.
plants AND animals - returns results only if they contain both the word plants and the word animals
plants OR animals - returns results that contain the word plants, results that contain the word animals, and results that contain both the word plants and the word animals
plants NOT animals - returns results that contain the word plants unless the item also contains the word animals (this operator is rarely used because it can easily exclude relevant results)
If you would be willing to accept multiple forms of a word, using * can save you time by pulling in variations of a stem word. For example, environment* will pull in results with environment, environmental, environmentalist, and environmentalism.