H P and R mean “heading, pitch and roll”. “heading” is also known as “yaw”. These are commonly known as “flight angles”; it is easy to visualise them using how they are applied using the analogy of an air plane:
So, a yaw/heading value of 90 means that the plane will face to the right, which is actually a rotation around the Z axis.
Also note that it matters in which order the H, P and R components of the rotation are applied. Pitching the airplane 90 degrees and then rolling it produces a different result to first rolling it and then pitching it!