This is a pint glass with a layer of lager on the bottom and a layer of stout on top. Bartenders make these using a perplexing combination of physics and magical spoons.
The name refers to a drink made by mixing a stout or porter and a pale ale (popularly Guinness and Bass); many bars will serve it in such a way that the stout sits above the ale, giving the distinctive black and tan look.
The drink black & tan is itself a reference to the term "Black and Tans" which refers to the Royal Irish Constabulary Reserve Force. They were one of two paramilitary forces employed by the Royal Irish Constabulary from 1920 to 1921, to suppress revolution in Ireland by targeting the Irish Republican Army.
The consumption message is a reference to how liquids of different densities form separate layers