Is this imperative in the usted form? Is that what it’s not “Toma”?
Yes, that’s it. One way I remember this is that the “usted” form is similar to the subjunctive, in that “-ar” verbs switch to an “e” whereas “-er” and “-ir” verbs switch to an “a”. The subjunctive is used to express uncertainty, and introducing uncertainty is an important component of politeness in most languages and cultures.
For example, in English, to take the simple and more casual or “ruder” command: “Do this.” and make it more polite you might say: “Would/could you do this?” English has fewer tenses, so it uses more “helping words” like “would”, but it’s a similar general idea.