Parentheses are an overlooked tool in BOOLEAN searching. When you build an advanced search string, it can be helpful to think of it like a math equation. As in math, parentheses indicated that 1) this should be done first and 2) the items inside are clustered together.
For example, I can make a search that say:
"Langston Hughes" AND "Harlem Renaissance" AND Impact OR Influence OR Importance
That will search for all my terms but gets a bit fuzzy at the end because Impact OR Influence OR Importance means that the Influence and Importance keywords could related to anything. (Databases are not as smart as Google).
If I change the search to say:
"Langston Hughes" AND "Harlem Renaissance" AND (Impact OR Influence OR Importance)
The parentheses now tell the database, I don't care which word inside the parentheses you use, but I want it attached to both Langston Hughes and the Harlem Renaissance.
Essentially, parentheses let you design a compound search to get more targeted and, thus, relevant results. It's a great way to clump similar keywords together.