Certain words, known in the information retrieval community as "stop words", are common in almost all English language writing. Most modern indexing engines eliminate these words from consideration during indexing, largely to pare down the size of the resulting indexes, and to improve semantic assessment of the concepts represented in each document.
SiteSurfer uses a fairly standard and well known list of stop words, but provides advanced users with the ability to add or remove words from the stop words list.