What is a spring tide and what causes this phenomenon?
A spring tide is when high tides are highest and low tides are lowest, whereas a neap tide is when the tides are evened out. See below for why tides change like this.
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.