What it means
The government agency responsible for UK vehicle registration records, registration certificates and vehicle tax.
For an imported vehicle, DVLA deals with the registration application and issues the V5C when registration is completed. It is a separate organisation from HMRC, which deals with import notification and tax, and from the bodies responsible for testing or approval.
When asking for help, identify the stage you are at: customs, approval or registration. That helps your question reach the right organisation. A private registration service can prepare paperwork and deal with correspondence, but DVLA remains responsible for the registration decision.