A VAT in general is a value-added tax... if you don't know what THAT is, it's similar to a sales tax, except it's added at intermediate points of sale, not just at the final transaction.
The UK VAT is therefore the VAT applied to purchases within the United Kingdom (of Great Britain and Northern Ireland) ... "England", to most USAans (though the UK includes also Scotland, Northern Ireland, and Wales as well as England proper).