A fiscal representative is a local tax agent or company that handles your VAT obligations. They are jointly liable for your VAT, so if you miss a payment, they may have to cover it. Fiscal representation is one of the methods EU countries use to prevent VAT fraud.
Not all EU States require non-EU businesses to have a fiscal representative. Where it is necessary, however, you won’t be able to register for VAT or submit a return without one.
Unless they have agreed to be appointed for customs matters, you shouldn't use your fiscal representative's address as the local address you declare at customs.