Dinosaurs, penguins and Baby Yoda: What are zero-g indicators?

If you watched the launch of the Crew-3 mission to the International Space Station at the end of October, you might have noticed an additional occupant inside the spacecraft alongside the four astronauts. It was a sequined toy turtle, chosen in recognition of two of the astronauts being the first members of their astronaut class (nicknamed 'the Turtles') to fly in space. It is a zero-g indicator.