Shallow[expr] prints as a shallow form of expr.
Shallow[expr, depth] prints with all parts of expr below the specified depth given in skeleton form.
Shallow[expr, depth, length] also gives parts whose lengths are above the specified limit in skeleton form.
Shallow[expr, depth, length, form] uses skeleton form for any parts which match the pattern form.
Omitted sequences of elements are given as Skeleton objects, which print in the form k.
In StandardForm, the characters used for this output are \[LeftSkeleton] and \[RightSkeleton].
Depth and length can be specified as Infinity.
Shallow[expr] is equivalent to Shallow[expr, 4, 10].
Shallow acts as a "wrapper", which affects printing, but not evaluation.
Trying to feed k as obtained from Shallow back as input to Mathematica in StandardForm will generate an error.
See The Mathematica Book: Section 2.8.5.
See also: Short.