HoldPattern[expr] is equivalent to expr for pattern matching, but maintains expr in an unevaluated form.
HoldPattern has attribute HoldAll.
The left-hand sides of rules are usually evaluated, as are parts of the left-hand sides of assignments. You can use HoldPattern to stop any part from being evaluated.
Example: expr /. HoldPattern[Integrate[y_, x_]] -> rhs transforms any subexpression of the form Integrate[y_, x_] in expr. Without the HoldPattern, the Integrate[y_, x_] in the rule would immediately be evaluated to give x_ y_, and the replacement would not work.
Example: f[HoldPattern[Integrate[y_, x_]]] := value can be used to make an assignment for expressions of the form f[Integrate[y_, x_]]. Without HoldPattern, the Integrate function would be evaluated at the time of assignment.
See The Mathematica Book: Section 2.5.6.
See also: Hold, Verbatim.