The republican proposal equalizes American taxes with much of the rest of the world with respect to imports and exports. Basically, in most countries, goods coming in (imports) are charged VAT while goods going out (exports) get a VAT rebate. Since the USA doesn't do that ATM, goods going into Mexico get VAT applied to them on the Mexican side but do not get rebated on the American side, while Mexican goods coming in get rebates from Mexico but nothing occurs in the USA beyond normal sales tax.

This is a different thing from taxing global profits, which is uniquely American (well, and Eritrea). VAT is something else.