Moss balls are a type of Algae found in several lakes in the northern hemisphere(Japan, Scotland, Japan, and Estonia). This form of algae is called Marimo. They are great at starving the algae in your aquarium and helping the water quality. They are interesting because they will float and sink depending when they need more light. Sometimes lumps will start forming on a moss ball, that's a new moss ball being formed. Some people don't want to wait and break their ball in a bunch of small balls.
Marimo tend to pick up debris in the tank, and should be gently squeezed to get all the water out, and rinsed in clean water before putting back in the tank. It's also good to take them out and roll them around your hand to maintain their sphere shape. In the lakes where they come from the current keeps moves them around and keeps the round shape that people love.