"Tell everyone that I won't be going tonight."
Here's a breakdown:
* Dile: This is the informal imperative form of the verb "decir" (to tell).
* a todos: means "to everyone"
* que: is the conjunction "that"
* esta noche: means "tonight"
* no iré: is the future tense of "no ir" (to not go)
So, the entire sentence means that the speaker is asking someone to inform everyone that they will not be attending something that evening.