VAT within EU transaction

Jason is a sole trader who has recently registered for VAT. He buys goods from a VAT registred trader in another EU state.

Why the answer says:
When Jason acquires goods from a VAT registrered trader in another EU member state, he is liable to VAT in the UK.
He will enter the transaction on his VAT return as an output and an input so the effect is neutral.

I do not understand why his VAT return will be an output and an input, and then becomes neutral.

Could anyone please explain this to me ?

Thanks very much.