To understand how the village of Salem (from Miller's play The Crucible) is representative of a crucible, one must first know what the different definitions of a crucible are.
First, a crucible is a place where a severe test or trial takes place. Another definition of a crucible is a place where something is subjected to extremely high temperatures. Both definitions fit to describe why Salem is an example of a crucible.
To explain, Salem is a place ruled by Protestant theology and ideology. All of the villagers of Salem are expected to follow the rules of the theocratic government (combining of church and state). If they do not adhere on their own, they are forced. Symbolically, they are put under fire until they adhere to the laws or leave.
As for the severe test, the witch trials were a place where the villagers of Salem were tested. Many were accused of witchcraft and had to face their accusers and the courts. If they submitted, they were allowed to live. If they did not, they were killed.