What is a spring tide and what causes this phenomenon?

The higher than normal tides that occur when the moon and are aligned.
Since both the moon and the sun have a gravitational pull on the earth, both create something of a tide. The moon is far closer and creates most of the tides on earth as the difference in gravity’s pull on one side is greater than the other.
The sun is far away and although it pulls harder on the earth than the moon (which is why we orbit the sun), the difference in pull from one side of the earth to the other is less due to the great distance. The sun contributes a small but measureable amount to the tides.
Spring tides occure when moon and sun are both working together to create higher than normal tides.