Normal[expr] converts expr to a normal expression, from a variety of special forms.
Normal[expr] converts a power series to a normal expression by truncating higher-order terms.
Normal[expr] converts RootSum objects into explicit sums involving Root objects.
When additional "data types" are introduced, Normal should be defined to convert them, when possible, to normal expressions.
See The Mathematica Book: Section 1.5.9, Section 3.4.3 and Section 3.6.5.
See also: SeriesCoefficient.