It's used in a few specific situations:
* After verbs like "suggest," "demand," "recommend," "insist," "request," "urge," etc. that express a desire or demand.
* In "that" clauses after verbs that express doubt, uncertainty, or possibility.
* In some formal expressions.
Examples:
* It is important that she be on time. (expressing a demand)
* I suggest that we be more careful in the future. (expressing a suggestion)
* It is doubtful that they be able to finish the project on time. (expressing doubt)
Important note: The subjunctive is less common in modern English than it used to be. In many cases, the indicative mood (using the regular forms of "be") can be used instead.