What do Y' and X'' mean in Euler angle convention in Mech-Viz

In Mech-Viz, the Euler angle convention of the robot is shown in the figure below, what do the marks ’ and ‘’ used with the Y and the X mean?

Z-> Y’-> X’’ is the sequence of the axes for three rotations. The mark ’ is used to distinguish the reference frame after a rotation from the reference frame before a rotation.
Y’ is the Y-axis of the mobile frame after the second rotation.
X’’ is the X-axis of the mobile frame after the third rotation.

The video below shows how the three rotations are performed: