Learn French: Mets ce peignoir et détends-toi. - Put on this bathrobe and relax.

When you give a positive command (telling someone to do something) using a reflexive verb like se détendre (to relax), the pronoun moves after the verb and attaches with a hyphen. Also, the pronoun te changes to toi. So, tu te détends (you relax) becomes détends-toi (relax!).

Why does mets have an 's' at the end? I thought we drop the 's' for the tu form of commands!

You only drop the 's' in the tu command form for verbs that end in -er (like mange or parle). The verb mettre (to put on) is an -re verb. Since its regular tu form is tu mets, it keeps the 's' in the command form.

Why do we use ce for "this" here?

Ce means "this" or "that" before a masculine singular noun that starts with a consonant, like peignoir. If the masculine noun started with a vowel, you would use cet, and if it were feminine, you would use cette.

Can mettre mean both "to put" and "to put on"?

Yes! Mettre is a very common and flexible verb. It can mean "to put" an object somewhere or "to put on" an item of clothing, as it does here with the bathrobe.