Boundless defines an isometric contraction as a contraction against a constant resistance. Basically, this means that the muscle is not changing in length or moving at the joints, but it is still contracting.
According to Education Portal, isometric literally means "same length". During an isometric contraction, the muscle contracts without changing its length. One example of isometric contraction involves holding a cup. The joints and muscles in the hand stay in one place, but the muscles are putting pressure on the cup so it does not fall. Isometric exercises are used for rehabilitation because they strengthen the muscles without placing stress on the joints.