Synergists and fixators are muscles that help the antagonist pair produce a more efficient movement.
A synergist may counteract an unwanted movement in the agonist.
A fixator holds the bone still that the agonist is attached to.
For example, during elbow flexion and extension the shoulder muscles hold the scapula (shoulder blade) still (both the biceps and triceps attach to the scapula).
The fixators hold the scapula still by increasing tension, but keeping the same length. In the same way that a guy rope holds a tent up.