preocupessin

form-of imperfect plural subjunctive third-person

これは『preocupar』の第三者複数形の未完了接続法（過去未完了接続法）の活用形です。

English Meaning

third-person plural imperfect subjunctive form of preocupar

Era important que no es preocupessin pels detalls menors.

小さな細部を心配しないことが重要だった。

It was important that they not worry about the minor details.