Sentence:  Sulley works by <WSD>fuzzing</WSD> the first fuzzable field to be <WSD>fuzzed</WSD>. While it is iterating through all the values it wants to try for that field, all the other fields are untouched and remain at their default value. 
Meaning of the word marked with <WSD> tags:  To test a software component by running it on randomly generated input.