It's used metaphorically to describe moods or situations that constantly fluctuate up and down, just like a yo-yo's movement. It's also used when talking about the stock market or weight changes.

The yo-yo is a toy with a long history, with examples found even in ancient Greece. In modern times, it's become more famous for the term 'yo-yo effect,' which describes regaining weight after a diet. As such, it's an interesting emoji with both positive and negative connotations.