In contrast to a compound exercise, an Isolation Movement stresses a single muscle group (or sometimes just part of a single muscle) in relative isolation from the remainder of the body. Isolation exercises are good for shaping and defining various muscle groups. For your thighs: squats would be a typical compound exercise. While leg extensions would be the equivalent isolation exercise.
Weight trainers tend to focus on compound exercises to build basic strength, whereas bodybuilders often use isolation exercises to visually separate their muscles, and to improve muscular symmetry. Specialized types of equipment are used to ensure that other muscle groups are only minimally involved—they just help the individual maintain a stable posture and movement occurs only around the knee joint. Most isolation exercises involve machines rather than dumbbells and barbells (free weights), though free weights can be used when combined with special positions and joint bracing.