Boolean operators, or search terms, are methods for combining searches. For example, you have performed 2 separate searches

1)    Angina Pectoris/
2)   exp "sensitivity and specificity"/

If you were really interested in citations about both angina pectoris AND it's diagnosis, you could enter in line 3) of your search:   1 AND 2.  This is termed a Boolean AND combination

If, on the other hand, you wanted to see all citations from both searches (which is unlikely) you would perform a Boolean OR combination:   type: 1 OR  2

You may however want to view the set of citations about angina excluding those that deal with diagnosis, a Boolean NOT combination.   Type 1 NOT 2
 

Or, if you are fimiliar with the specific search notations you can type in the entire sequence at once.  This saves a lot of time.

Note thate the notation for searching is somewhat different in Grateful Med and PubMed