Learn French: Ils ont une représentation dans un mois. - They have a performance in a month.

While dans usually means "inside" a physical space, it is also the standard word to say "in [a certain amount of time] from now". Here, dans un mois means exactly "in one month" or "one month from today".

I thought représentation meant "representation". Why does it mean "performance" here?

Représentation is a false friend! While it can mean "representation" in some contexts, in the performing arts (theatre, dance, etc.), une représentation translates to "a performance" or "a show".

Can I use the future tense here instead of the present tense ont?

Yes! You could say Ils auront (They will have). But just like in English, French often uses the present tense for scheduled future events. The phrase dans un mois makes the future timeframe obvious, so the present tense works perfectly.

What is the difference between saying dans un mois and en un mois?

Dans un mois tells you when an event will happen (one month from today). En un mois tells you how long it takes to complete an action (e.g., "I learned this song in a month"). Because we are talking about the future date when the performance happens, we use dans.