There are various types, but in general terms it's a pump to which you attach an ordinary garden hose that sits on top of your solid winter cover and automatically pumps rainwater off as long as the electrical cord is plugged in - you don't need to be home to monitor. I've tried several kinds over the past 7 years and have found my favorite is the Rule cover pump ($150). It will automatically turn on every 5 minutes and if it senses water will pump it off. However it's a rather lightweight pump (but pumps at a high velocity) so with torrential rains I'll ultimately have to use my pool brush to brush deep water towards the pump to reduce standing water but that's no bother; and there's no problem unattended because if the water gets too deep some will always gravitate towards the pump so the water will not get so deep as to displace pool water under the cover.

Worst pump I ever tried was Little Giant's Water Wizard. It operates on the float-principle, i.e., when the water reaches a certain depth and activates the float it automatically turns on and pumps water. But it has a large flat plastic base and is not horizontally stable on the cover in deep rainwater and as well is very difficult to clean out the clogs as you have to unscrew the base plate (it's very easy to unclog with the Rule pump as the motor just snaps right into the base). Moreover, I'm quite sure the sharp plastic base edges tore my cover one year. I love the Rule automatic pump.