A "crucible" is a container used for producing materials in which very high temperatures are used. Think of it as a place where high temperatures (pun on tempers) are produced during a chemical reaction (over-"reaction" of religious zealots in "The Crucible").
A crucible is also defined as a test. And this is symbolic because those put on trial are tested. But more to the overall point of the play, the entire town is morally tested; a test they collectively failed, having gone through with all the arrests and executions.
The title works for these two reasons. It is a place of hot, concentrated, interaction which produces some new (for better or worse) product. And a crucible is a test: of morality, truth, and justice.