a monopod allows you to use slower shutter speeds by supporting your camera. But the situation depends on how slow you need to go. As it has only 1 leg, it doesn't support the camera by itself; rather uses the photographer to help steady the camera. That being said, it can still be prone to handshake when you do below a certain threshold.
It's still not as stable as tripod so i don't think you can use it for very slow shutter speed in night photography..
As they said monopod is a way for extra support on top of the one given by the photographer. How to use it? Usually I will use my both legs as 2 legs of a tripod with monopod as the third leg.
It is useful at times when you do not need very slow shutter speed but need extra support under space constraint and easier to carry ard. E.g shoot sports using telephoto when it is hard to spread out legs of tripod.
Not sure what to get? Get those tripod with detachable legs to use as monopod.