While this question has been posed many different times and different ways on eNotes The answer here will provide a different view from the other answers posed.
The term "crucible" means (according to Google dictionary):
1. A ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures.
2. A place or occasion of severe test or trial.
Both of the definitions can be used to explain the title of the play and the events which are depicted.
First, the community itself is subjected to pressure, or "heat", based upon the Puritan theology the society upheld. The people of the community were expected to fit into a very specific mold as defined by Puritan culture and ideology.
The second definition examines the more specific tests that the people of Salem faced in regards to the accusations made by others. Many people were accused of witchcraft and was forced to face test and trial. The 'occasion' of the witch trials forced the people of Salem to face the trails of their lives. As proven by both Proctor and Giles Corey, the people were fighting for their lives.
One last way to look at the use of the word crucible in the title of the play is that, historically, people facing the charges of witchcraft could undergo a trial by fire. Here, one can see where the first definition influences the use of the word.
Other views on this topic are offered in the reference links below so that you may examine them as well.