Isometric exercises are contractions of a particular muscle or group of muscles. During isometric exercises, the muscle doesn't noticeably change length and the affected joint doesn't move. Isometric exercises don't effectively build strength but can help maintain muscle strength — most often in a rehabilitative setting.
Because isometric exercises are done in one position without movement, they'll improve strength in only one particular position. You'd have to do various isometric exercises through your limb's whole range of motion to improve muscle strength across the range. In addition, since isometric exercises are done in a static position, they won't help improve speed or athletic performance.
Isometric exercises may be helpful to someone who's been injured or has a condition such as arthritis, which could make movement painful or be aggravated by using muscles to move a joint through the full range of motion. For instance, if you injure your rotator cuff, your doctor or physical therapist might initially recommend isometric exercises involving the group of muscles that helps stabilize the shoulder to maintain shoulder strength during recovery.
It's also important to note that isometric exercises generally aren't recommended for people who have high blood pressure or heart problems, because the large increase in muscle tension caused by isometric exercises can dramatically increase blood pressure.