What is a discretionary clause?
A single sentence in your plan document that decides whether a court reviews your denial fresh or only for reasonableness.
A discretionary clause is language in a plan document giving the plan administrator or claims fiduciary the authority to interpret plan terms and decide eligibility for benefits. Under Firestone Tire & Rubber Co. v. Bruch, that single clause is what shifts a court's review of your denial from deciding the claim fresh (de novo) to only checking whether the administrator's decision was reasonable, a much more deferential and claimant-unfriendly standard. See standard of review: de novo vs. arbitrary and capricious for what that difference means in practice.
The Department of Labor's own 2016 disability-claims rulemaking record notes that commenters specifically raised "the validity of discretionary clauses in plans that are used as a basis for seeking a deferential 'arbitrary or capricious' standard for court review of benefit denials" as a contested issue, which tells you this is a real, actively disputed feature of ERISA practice, not a settled technicality.
Where to look for it
Look in your plan document or Summary Plan Description for language naming who has "authority to interpret," "discretionary authority," "final and conclusive" decision-making power, or similar phrasing, usually in a section describing the plan administrator's or claims fiduciary's powers. If you cannot find or understand this language, an attorney who handles ERISA LTD appeals can identify it quickly from your plan document.