What is a spring tide and what causes this phenomenon?

This answer gives a good explanation of spring and neap tides. Basically: Tides are caused by both the Moon and the Sun. When they both fall in line with Earth, in a new or full Moon, they work together to make high tides higher and low tides lower. That is the spring tide. When the Moon and Sun for a right angle with Earth, they fight each other and the tides are more evened out, creating a neap tide.

A spring tide is a type of tide that occurs during the full moon and new moon phases. It is characterized by the highest high tides and the lowest low tides. This phenomenon is caused by the gravitational alignment of the Earth, Moon, and Sun, creating a strong gravitational pull on the Earth's oceans. During a full moon or new moon, the gravitational forces of the Moon and the Sun combine, resulting in increased tidal range.