In order to retrieve relevant results, it is best to group synonyms in parenthesis with a capital OR (to capture different phrasing of the same concept) and connect different concepts with a capital AND (to capture only results that include all of your concepts together). These are known as Boolean operators.
To improve precision in keyword searching, consider using field tags, which specify which parts of the record to search. For example, a search for kidney[tiab] would retrieve all records that include the word “kidney” in the title or abstract fields, and would not automatically apply any additional terms.
To search for specific phrases in PubMed (rather than individual words) put quotation marks around the phrase you want.
A PubMed search for "medical marijuana" will retrieve a smaller, more focused set of results than a search for medical marijuana.
You can use truncation to search for multiple possible endings to a partial word, but ending the partial word with an asterisk (*).
For example, a search for child* will return searches for children, childhood, and other terms beginning with "child".
Note that both phrase searching and truncation will disable automatic term mapping and MeSH terms will not automatically be applied to the search.