It's a question used to inquire about the timing of a future event or situation.
Here's a breakdown:
* Cuando: When (interrogative adverb)
* será: future tense of "ser" (to be)
* eso: that (pronoun)
Examples:
* "¿Cuándo será eso?" - When will that be? (asking about a specific event)
* "No sé cuándo será eso, pero espero que sea pronto." - I don't know when that will be, but I hope it's soon.