Learn Italian: Arrivo tra un'ora. - I arrive in an hour.

Tra (and its twin fra) usually means "between" when talking about physical space. However, when you use it with a period of time, it means "in" that amount of time from now. So, tra un'ora means "in an hour".

Could I say fra un'ora instead?

Yes, absolutely! Tra and fra mean exactly the same thing and are completely interchangeable. Italians sometimes pick one over the other just to avoid repeating sounds, but both are always grammatically correct.

Why is there an apostrophe in un'ora?

The word for "hour" is ora, which is a feminine noun starting with a vowel. The feminine word for "a" or "an" is una. When una is followed directly by a vowel, it drops its final "a" and takes an apostrophe to make it flow better when speaking. So, it becomes un'ora instead of una ora.

Why use the present tense arrivo (I arrive) if the action is happening in the future?

In English, we would usually say "I will arrive" or "I am arriving". In Italian, it is extremely common to use the simple present tense for near or planned future events. Because the phrase tra un'ora already tells us this is happening in the future, the present tense arrivo is all you need.