# What is the unit $P_s$ in this chart about quantum spin oscillation in a magnetic field?

I came across this in an 'Up and Atom' video on YouTube, hosted by Jade Tan-Holmes. It is the third video in her quantum-biology series, about bird navigation, at 13:30 of the video....

The one axis is obviously just time, measured in microseconds....

But what exactly is the other axis?

$$P_s$$ (t) I don't get it....