One's strong point or specialty; the area someone excels in (used also in the phrase お株を奪う — to take someone's specialty/credit).
料理は彼女のお株だから、パーティーのメニューはすべて任せた。
Cooking is her forte, so I left the party menu entirely to her.
彼のジョークで上司のお株を奪ってしまった。
His joke stole the boss's thunder.
この分野は部長のお株なので、意見を聞いてから動きましょう。
This area is the manager's specialty, so let's act after asking for his opinion.