___ (three _ characters) or BlankNullSequence[ ] is a pattern object that can stand for any sequence of zero or more Mathematica expressions.
___h or BlankNullSequence[h] can stand for any sequence of expressions, all of which have head h.
Blank sequences work slightly differently depending on whether or not the head of the expression in which they appear is a symbol with the attribute Flat.
Consider matching the pattern f[, , ... , ___, , ... ] against the expression f[, , ... , , ... , , ... ]. If f is a symbol with attribute Flat, then the ___ will be taken to stand for the expression f[, ... ]. If f is not a symbol with attribute Flat, then ___ will be taken to stand for the sequence of expressions , ... . With a named pattern, such as x___, x can be used only as an element in an expression. The sequence of expressions , ... is "spliced in" to replace x, thereby usually increasing the length of the expression.
If ___ matches a sequence of length more than one, then the sequence will be represented by a Sequence object. In most uses of ___, however, the Sequence object will automatically be spliced into another expression, and will never appear explicitly.
See The Mathematica Book: Section 2.3.8.
See also: Pattern, SlotSequence.