A neap tide is when there is the least variation between high and low tides. This happens twice in a lunar cycle, when the moon is a first quarter moon and when it is a third quarter moon.
Tides are affected by both solar and lunar gravitational pulls. When the sun and moon are aligned with the earth (a new moon) or when the sun and moon are on opposite sides of the earth (a full moon), the tidal forces are accentuated resulting in a large variance between high and low tide. This is called a spring tide. In between these times, especially during the first and third quarter moon, the lunar gravitational pull cancels out some of the effects of the solar gravitational force and the tides are less pronounced. This is the neap tide.