The International Polar Motion Service is like a group of scientists who study how the Earth moves around. Imagine that the Earth is like a big ball that spins around in space. Sometimes, this spinning ball wobbles a bit, kind of like a toy top that starts to get wobbly when it slows down.
The scientists at the International Polar Motion Service watch this wobbling very carefully because it can affect things like GPS satellites, which help us find our way around. They use really special equipment to track how the Earth's spin changes over time, and they share this information with other scientists all over the world so everyone can understand how our planet is moving.
So, in simple terms, the International Polar Motion Service is a group of people who watch how the Earth spins and wobbles, and they share what they learn with everyone else.