This is the 2143rd most frequent Spanish word.
debían
"They owed" or "they had to" (imperfect tense of "deber").
In this sentence, 'debían' (they had to) is used to indicate an obligation or duty in the past.
Ellos debían terminar el proyecto antes de la fecha límite.
They had to finish the project before the deadline.
'Debían' here reflects an expectation or recommendation in a past situation.
Los estudiantes debían estudiar para el examen, pero no lo hicieron.
The students were supposed to study for the exam, but they didn't.
In this case, 'debían' implies a suggestion or advisement in a past context.