I'm not sure what "power signal" is in terms of PSU without googling, but hold time is the time the PSU will continue to provide power when it's unplugged. The longer the better, especially for a critical system.
When buying a UPS, or more specifically building one, it's crucial to look at the transfer time of whatever you are using (relay, inverter-charger etc) so that it is lower than the hold time of the PSU(s) that will be connected to it.
A prebuilt UPS is designed for computers so normally their transfer time will be like <4ms. But if you buy an inverter-charger it may be like <15ms, I've even seen some that are 2 seconds, which is worthless for computers, maybe ok for motors. So you want to ensure any power supply connected to it has a hold time higher than the UPS or other backup equipment you have.