p.. is a pattern object which represents a sequence of one or more expressions, each matching p.
p.. can appear as an argument of any function. It represents any sequence of arguments.
All the objects in the sequence represented by p.. must match p, but the objects need not be identical.
The expression p may, but need not, itself be a pattern object.
See The Mathematica Book: Section 2.3.11 and Section A.2.7.
See also: RepeatedNull, BlankSequence.