Tides change as the relative position of the moon and sun changes. The amplitude, or range, between any two successive high tides (and low tides) gradually increases from some minimum height to a maximum value and then decreases back to a minimum.
Periods of maximum amplitude are referred to as spring tides; times of minimum amplitude are neap tides. This envelope of spring to neap occurs twice over a period of approximately 29 days.
Spring tides occur when the sun and moon are in alignment. This happens at either a new moon, when the sun and moon are on the same side of the earth, or at full moon, when they are on opposite sides of the earth. Neap tides occur at the intermediate points, the moon's first and third quarters.