Learn French: Vous comprendrez que les citoyens ont ce droit. - You will understand that the citizens have this right.

It comes from the regular -re verb comprendre (to understand). To form the future tense, you simply drop the final e to get the stem (comprendr-) and then add the future ending for vous, which is -ez.

Why do we use ce before droit instead of cet or cette?

The word droit is a masculine singular noun starting with a consonant, so it takes the demonstrative adjective ce (meaning this or that). You would use cet if it started with a vowel sound, and cette if it were a feminine word.

Is droit related to the direction right?

Yes! Just like in English, the noun un droit means a legal or moral right. The word is also used to mean the direction right (as opposed to gauche) or the concept of being straight.

Why is ont in the present tense if the first part of the sentence is in the future?

The sentence describes an action happening in the future (comprendrez), but what you will understand is a current, ongoing truth (the citizens possess this right right now). Therefore, ont (from the verb avoir) remains in the present tense.