All tides, including the cycle of spring and neap tides, are caused by the gravitational pull of the sun and moon on Earth's ocean tides. Each month, the angles and alignment of the sun, moon and Earth work to influence the cycle of tides.
Twice a month, during the full and new moons, the sun, moon, and Earth are in alignment with one another and the solar tide works together with the lunar tide, creating very pronounced high and low tides. This type of tidal pattern is called spring tide.
During the alternating weeks, the sun and moon are at right angles to one another so the solar tide partially cancels the lunar tide, resulting in a very moderate high/low tide cycle. This pattern is called neap tide and occurs twice monthly.