It’s ‘Britain’, a geographical rather than administrative/political/constitutional term, which is confusing. The border is that of the United Kingdom. It consists of a sea border between England/Wales/Scotland and all other states, plus a land border between Northern Ireland and the Republic of Ireland. Great Britain (England, Wales and Scotland), together with Northern Ireland, form the United Kingdom.
And that presents a big problem. The referendum was for maximum brexit, which means taking Northern Ireland with them. But, putting a land border through Ireland violates the Good Friday Agreement and re-ignites a war with the IRA and seriously damages relations with Ireland. If they leave Northern Ireland behind, they violate the referendum and create a template for Scotland leaving the UK since they don't want to leave the EU.