Nurdles are very small pellets of plastic which are used as the smallest building block in manufacturing plastic products. Every plastic toy, tub, and toothbrush starts its life as a nurdle. Due to poor handling, deliberate dumping, and spillages, these tiny little menaces end up in the sea en mass.
As they float on water, nurdles end up dotted across our coastlines and beaches. As they're so small, they're often mistaken by animals for fish eggs, or food. Because of this, they end up inside fish, birds, and other animals; and they seep harmful chemicals into the water that we drink and swim in.
And, if it's not obvious, that's bad.