The materials we use in a standard darkroom are quite sensitive to some colours of light, and not very sensitive to other colours of light. A safelight is a light that emits a colour of light which most papers in the darkroom are not very sensitive to.
So when you look at the specifications of a light-source you hope to use as a safelight, you check what the central frequency is of the light emitted by that light-source and, if you can, how sharp the cut-off is of that source. Then you compare that information with the specifications for the photographic paper you are using.
Most papers here in North America are designed for use with a light filtered by something like a Kodak OC filter. That has a peak transmission for light at about 580-590 nm.