Learn Russian: Завтра они уезжают за границу.

Just like in English ("they are departing tomorrow"), Russian can use the present tense of an imperfective verb to talk about a scheduled event or fixed plan in the near future. You could also use the perfective future tense (уедут), but the present tense highlights that the plan is already set in motion.

What does the prefix у- in уезжают add to the meaning?

The prefix у- indicates "departure" or moving "away." When attached to the base verb for riding or driving, it creates уезжать (to leave, to depart by vehicle). It shifts the focus from the journey itself to the act of leaving a location.

Why does "abroad" translate to two words (за границу)?

Russian doesn't have a single equivalent word for "abroad." Instead, it uses a phrase that literally means "beyond the border." Граница means "border" or "boundary," and the preposition за means "behind" or "beyond."

Why is it границу (accusative case) rather than the instrumental case?

Because this sentence is about motion towards a destination. When the preposition за shows direction or motion, it requires the accusative case (за границу). If they were already located abroad (answering "where?" rather than "where to?"), you would use the instrumental case: за границей.