Boro Mods are vape devices that use the Boro Tank System instead of the usual 510 thread connection.
The word Boro got its name from borosilicate glass, which was used in the original Billet Box's tank as the sliding cover. "R4" means "Billet Box REV. 4" so sometimes you might see the term "R4" on a boro product. 
Nowadays, Boros have exploded in popularity, and there are lots of different options to choose from.