Learn French: Vous devriez laisser un pourboire sur la table.

Devez is the present tense and means "you must" or "you have to." Devriez is the conditional tense. In French, we use the conditional of devoir when giving advice or a suggestion, which translates perfectly to "you should" in English.

Why isn't the verb laisser conjugated?

In French, when you have two verbs back-to-back, the second verb stays in its infinitive (dictionary) form. Since devriez is already conjugated, laisser (to leave) is left unchanged.

Does the word pourboire literally mean "for drinking"?

Yes! It is made of the words pour (for) and boire (to drink). Originally, it was a few coins given to a worker so they could buy themselves a drink. Today, it is simply the standard French word for a tip.

If I were talking to a friend, how would this sentence change?

You would use the informal "you" (tu) instead of the formal or plural vous. The conditional form for tu is devrais, so the sentence would become: Tu devrais laisser un pourboire sur la table.