Having selected a set of keywords to use for your search, you can choose how those terms should be evaluated. An 'OR' search will combine results for each keywords, whereas an 'AND' search will yield only those opportunities that contain every selected term.
'OR' Example: If you have two keywords, 'Blue' and 'Green' and you use the 'OR' operator, the result would be the union of all opportuniities that contain either A or B keywords. A+B+C in the Venn diagram shown here.
'AND' Example: Using the same example, the result would be the instersection of opportunities with both A and B. Only C in the Venn diagram shown here.