WorkingPrecision is an option for various numerical operations which specifies how many digits of precision should be maintained in internal computations.
WorkingPrecision is an option for such functions as NIntegrate and FindRoot.
Setting WorkingPrecision->n causes all internal computations to be done to at most n-digit precision.
Even if internal computations are done to n-digit precision, the final results you get may have much lower precision.
See The Mathematica Book: Section 3.9.3 and Section 3.9.3.
See also: AccuracyGoal, Precision, Accuracy, N.