If it's just a project, all it means is set the power supply to 24V but it'll probably be fine from anywhere between 20V and 28V but it's difficult to say without seening it.
If you have to ask, it means that the writer was not making himself clear.
There is no range of acceptable voltage given. eg: 24.0V +/- 0.1V
I imagine that it means that you should check the adjustable PS voltage with a DMM and adjust is as closely to 24V as possible. It may have been adjusted to another voltage previously if it is a universal bench power supply. If it is a dedicated supply, the voltage may have drifted out of specification.