This specification describes the range of AC input voltage frequencies over which the supply is designed to operate. Put differently, it’s the range of power line frequencies over which the rest of the specs given are considered valid and applicable. It’s not really a direct statement of a device’s capabilities or behaviors, so much as it’s a declaration of the expected use conditions that the manufacturer decided on when designing the product. Operation outside these limits is quite possible, but doing so is at one’s own risk; the device may not perform as described or it may fail, and manufacturers generally won’t provide technical support or warranty service for product used outside of specified limits.
It depends on the internal design of the supply, so loose generalities are all that can be offered. Older/simpler designs often have an input stage consisting mainly of a bridge rectifier and a filter capacitor, and such supplies will often operate quite satisfactorily from a DC source. Designs incorporating power factor correction (PFC) are more varied, so their behaviors with DC or otherwise out-of-spec inputs are less predictable.
Problems with low frequency AC inputs are largely due to the need to store sufficient energy to deliver power through the low points in the AC waveform. AC-fed supplies are commonly designed with enough internal energy storage to deliver full output for some milliseconds (the “hold up time”) after removal of the input voltage, and conceivably could be used at below-spec frequencies with reduced loads. Doing so can stress the input filter capacitors well beyond what was contemplated by the original design however, and rapid failure or a shortened service life could be expected.
Above-spec line frequency inputs might cause a wide variety of issues, ranging from excessive device stress due to increased switching losses or parameter shifts with ripple frequency, conflicts with frequency/bandwidth of various filters or control loops, and probably a dozen other items. Generally speaking though, the maximum input frequency spec on an AC-DC converter can tolerate being fudged a bit, particularly with the simple-input types. A device rated for a maximum input frequency of 63 Hz is likely to work just fine at 64 Hz. But 64 kHz? Probably not.