It signifies a baronet (traditional abbreviation Bart, modern abbreviation Bt) or the rare female equivalent, a baronetess (abbreviation Btss).
This person is the holder of a hereditary title awarded by the British Crown known as a baronetcy.
Like knights, baronets use the title "Sir" before their name. Baronetesses in their own right use "Dame", while wives of baronets (though legally a Dame) use "Lady" by longstanding courtesy.
Notable baronets:
Sir Robert Baden-Powell, 1st Bt (founder of the world Scouting movement) (1857-1941)
Sir Samuel Cunard, 1st Bt (shipping magnate)
Sir Denis Thatcher, 1st Bt (businessman; husband of Margaret Thatcher)
https://en.wikipedia.org/wiki/Baronet
