Obligation or Advisability

The verb devoir (to have to, to be supposed to) expresses primarily obligation:

Je dois préparer le repas pour midi.
(I must prepare the meal for noon.)

In the conditional, devoir means: ought to, should:

C'est son anniversaire; je devrais lui téléphoner.
(It is his birthday; I should/ought to call him.)

Vous nous manquez: vous devriez venir plus souvent.
(We miss you: you should visit more often.)

When followed by a noun, devoir means to owe:

Vous me devez dix francs.
(You owe me ten francs.)

The expression: faire mieux + de + infinitive is also used to denote advisability: This form is used in the conditional.

Vous feriez mieux d'écouter si vous voulez apprendre quelque chose.
(You had better listen if you want to learn something.)