More info about principal lunar semi-diurnal constituent. Basically it is the tide that the Moon causes twice everyday. The tide can be larger or smaller due to other factors, e.g. varying distance between Moon and Earth, or when the Sun creates spring or neap tide.
I know.
See image.
There’s other factors this is one component of the tide. The lunar tide can be magnified by the shape of the floor, like if there’s a shelf close to shore