Brake drag is when a brake stays partly applied and keeps rubbing the rotor even when your foot is off the pedal. A tiny amount is normal; noticeable drag is a problem. Here is what brake drag is and what causes it.
What drag means
Ideally the pads pull back a hair from the rotor when you release the brake so the wheel spins freely. Brake drag means the pad stays in contact and creates constant friction - wasting energy, making heat, and wearing the pad even when you are not braking.
Common causes
Drag comes from a sticking caliper or seized guide pins - see sticking caliper - a collapsed brake hose acting like a one-way valve, corroded pad slides, or an over-adjusted parking brake. Rust in the caliper bracket that binds the pad is a frequent culprit; see brake caliper bracket.
Why it matters
Constant drag generates heat that can fade the brakes and warp the rotor; see why brakes get hot. It also cuts fuel economy and wears the dragging pad quickly. Bad drag on one wheel makes that corner noticeably hot.
How to fix it
Track down the binding part - free or replace the caliper, service the slides and clips, or replace a bad hose. Clean and grease the pad contact points; see brake grease points. Fit fresh pads and hardware.