Unevaluated[expr] represents the unevaluated form of expr when it appears as the argument to a function.
f[Unevaluated[expr]] effectively works by temporarily setting attributes so that f holds its argument unevaluated, then evaluating f[expr].
Example: Length[Unevaluated[5+6]] .
See The Mathematica Book: Section 2.5.5.
See also: Hold, HoldFirst, ReplacePart.