A stopper knot is often used to prevent rope from unexpectedly going throu some hole, such as a belay device.
How would I tie a stopper knot?
A stopper knot is not a specific knot, but a technique fo preventing a rope from sliding through a loop or hole. Common knots used for a stopper knot are:
You need to use a figure-eight knot, because if it is weighted you can still untie it. There are lots of knots to use, but figure-eights are the most common for this reason.