Neap tides, moderate tides resulting from the gravitational forces of the sun and moon partly canceling out, happen twice every lunar month. A neap tide happens exactly half-way between a full moon and a new moon.
At the full moon and the new moon, the moon, sun and Earth align, an occurrence referred to as a syzygy. This causes a "spring tide," during which the high and low tides have a large tidal range, the difference between high and low tide.
During a neap tide, the sun and moon are at right angles to the Earth, and instead of amplifying one another as with a spring tide, the actions of lunar and solar tides interfere with one another. This results in a small tidal range.