It means “shouldn’t”. It’s funny because shouldn’t and mustn’t can be used interchangeably in French without too much confusion. For the sake of correct grammar however, “ne devait pas” would be the equivalent for “shouldn’t.”

On the other hand, “ne devrait pas” would be the equivalent for “mustn’t.” It’s the same verb, but in a different tense.

Should is devrait. Conditional with devoir basically means “I can’t tell you what to do, but it would be better if you …” = “you should …”

Indicative (imparfait here) with devoir means must (*). It’s a strong obligation (or a forbidding with a negative).

(*) Both for obligations and sentences like “He must have forgotten” (Il a dû oublier)