The ETSO (European Technical Standard Order) authorization represents one way (and not the only way) to have parts and appliances approved. This is an optional step which ensures that a part or appliance complies with a minimum performance standard. In all cases, the installer must apply for an installation approval on-board the aircraft. He/she can use the ETSO authorization and he/she complements it to demonstrate that the installation complies with the applicable certification basis for the changed aircraft.
European Technical Standard Order authorizations are covered by Subpart O of Part-21 which is the Annex to Commission Regulation (EU) No 748/2012. You can find a list of the ETSO standards in the published list ETSO Index. The certification specifications for ETSO are published here.