Well, it means the first partial derivative of the function Pattern with respect to the first variable, which is probably not what was intended.
Probably this came about due to an input similar to D[Sin[?_], ?]. Since the FullForm of Sin[[ ?_] is Sin[Pattern[\[ Theta], Blank[]]] and we are asking for a derivative with respect with ?, Mathematica applies the chain rule and obtains Cos[\[ Theta]_]*Derivative[1, 0][\[ Theta], _].
