Nesting is using parentheses ( ) to ensure that Boolean operations are performed in the sequence you intend. Nesting synonyms can broaden your search. Nesting is typically used when you have only one search box. Most databases use parentheses but not all.
(smoking AND (teenagers OR adolescents))
will retrieve both of the following:
Smoking is on the rise in teenagers
Adolescents turn to smoking to relieve stress