Isolation exercises are movements that work one joint and target one muscle at a time. Biceps curls, lateral raises, leg extensions, cable flyes. Because nothing else is helping, all the load goes to the muscle you're aiming at. That makes them the opposite of compound exercises like squats and rows, which move multiple joints and recruit several muscle groups at once.
Why it matters
Compounds build the most muscle per minute of gym time, but they have blind spots. Side delts, biceps, calves, and rear delts rarely get pushed hard by big lifts alone. Isolation work fills those gaps. It also lets you add volume to a lagging muscle without piling more fatigue on your spine, hips, and nervous system. A set of curls costs you almost nothing in recovery. A set of heavy deadlifts costs plenty.
The catch? A program built mostly from isolation moves is slow going. Curls won't grow your back.
How to use it in training
Do your compounds first, while you're fresh. Then finish with two to four isolation exercises aimed at whatever you want to bring up: lateral raises for shoulders, curls and pushdowns for arms, leg extensions or hamstring curls for legs.
Moderate weight, higher reps (10 to 20), and a real squeeze at the top work well here. Chasing heavy singles on a cable fly is a shortcut to cranky joints. And if a muscle already grows fine from your compound work, you can skip the extra isolation for it. That's not laziness, that's smart budgeting.