"Where are you going to travel to next year?"
Translation:¿Adónde vas a viajar el próximo año?
Adónde is expressing movement - TO where are you going, as opposed to donde which would just be asking where you are. I find the difference quite subtle, as an English speaker, and it takes time to get your head around and remember to use it.
Donde means - where Adónde means - to where Donde refers to - location Adónde refers to - direction Donde is used to ask- where someone or something is located Adónde is used to ask - where someone or something is going Donde is generally followed by a noun or preposition Adónde is generally followed by a verb signifying movement
in answer to why 'a' is needed, it is used to signify going to do something in the near future, near future being subjective, as in this example when talking of next year.
Ir + a + infinitive = expression of doing something in immediate/near future.
I find that, more than anything, without the 'a' the sentence doesn't flow but I realise that is not an explanation!
hopefully the above helps