That shows the tidal range on any given day between low and high tide. Tides differ in range over the course of a month, depending on the pull of the moon. A Neap tide occurs on the day in the cycle with the lowest tidal range. (Bonus fact: Neap is derived from the Nordic word for plenty). These days see the highest low tide and the lowest high tide, so effectively, there’s generally some water around. The movement of the tide is fairly benign on these days as there’s less water to move between the low and high water marks.

Conversely Spring tides see the highest high tides and the lowest low tides. Movement of water is therefore far greater on these days.

There are 2 Spring tides and 2 neap tides in every month.

You may see this expressed as Neaps-2d, Neaps -1d, Neaps, Neaps +1d, Neaps +2d, Neaps + 3d, Springs -4d, Springs -3d etc. as the tidal phases are counted down and up.

Hope this helps.