Databases are programmed to prioritize Boolean Operators, (AND, OR, NOT and even more complicated ones) This is often called "search precedence" and it influences how your search is performed.
For instance, most databases give AND precedence over OR, meaning terms linked with AND will be searched before those linked with OR, no matter the order they appear in your search.
In any database, using parentheses will override the search precedence.
Examples of Boolean searches using parentheses:
(children OR kids) AND nutrition
Notice that the OR links two synonyms
(elderly OR "older adults") AND (computers OR technology)
You can use two or more sets of parentheses
((teenager OR teen OR adolescent) AND media) NOT television
You can also use parentheses within parentheses.
Notice the effect of the position of the keywords inside parentheses.