What does the * (asterisk) next to radio and nav aid frequencies on Jeppesen charts refer to?
It means that this frequency is not always available (not always staffed). From the Introduction to Jeppesen Navigation Charts:


8 - Indicates the service is part time.

And further below:

Navaid boxes include the navaid name, identifier, Morse code, and frequency. A letter &quot;D&quot; indicates
DME capability with an asterisk indicating part time.