Unprotect[, , ... ] removes the attribute Protected for the symbols .
Unprotect["", "", ... ] unprotects all symbols whose names textually match any of the .
A typical sequence in adding your own rules for built-in functions is Unprotect[f]; definition; Protect[f].
See notes for Protect.
See The Mathematica Book: Section 2.4.12 and Section A.3.12.
See also: Protect, Locked, SetOptions.