"motivate oneself and persist in the face of frustrations; to control impulse and delay gratification; to regulate one's moods and keep distress from swamping the ability to think; to empathize and to hope."
In other words, it's the ability to maintain a positive attitude and emotional control in the face of what life throws at you. Mark showed plenty of this kind of intelligence in his brushing off of snide comments from colleagues. Goleman argues that intellectual ability, or competence, can't by itself predict success. Intellectual ability plus emotional intelligence is what it takes. Failure to manage your emotions appropriately can both stunt your ability to learn and impede your ability to make use of what you do learn.
Read more here.