IOSS is the 'Import One Stop Shop\ a pan-european / EU arrangement allowing all businesses in the EU to collect VAT at the relevant NATIONAL rate, and remote all the returns as a blob to their local tax agency, who then sort it out amongst themselves. Previously you had to register for VAT in every EU state you hit the threshold in, now you can just register in Ireland, and collect and remit VAT for every EU state (customs across the EU probably have noticed our sales now charge ES TVA, or French IVA, etc)