When you set out rat poison and the mice eat the rat poison they then go back to their nests or wherever they were hiding and then die there.
The poison can take around 24 to 48 hours to actually kill the mice or rat so it won't kill the mice or rat immediately.
Although Rat Poison can kill the mice and rats it usually is not a good option for getting rid of mice or rats since they go off, die and then decompose and start to smell.
So you then have to search for where the smell of the dead mice or rat is and remove it which could possibly be behind walls or in the attic and then get rid of the mice or rats to get rid of the smell.
A better option is glue traps, or snap mouse traps that you bait with food that kill the mice almost instantly and then you don't have to worry about the smell.
Sometimes though there's no other option if the mice or rats are avoiding the glue or snap traps and then the rat or mice poison becomes the only other option.
Or you could also make a mouse trap with some water and a bucket and a plastic bottle that spins as the mice walk across it and then falls into the bucket and the dies.