Roast a turkey by cooking it in the oven. Roast a friend by calling her a turkey on her birthday. To roast can mean to cook, or to poke fun of someone as a way of honoring them.
In cooking, slow, dry heat is used to roast things. When a cook roasts a piece of meat, you can call the meat itself a roast too. Another way to use this word is to mean “to make fun of someone,” like a toast but meaner and funnier. Comedians are known to roast each other, telling one joke after another mocking the person they're celebrating. This comes from an eighteenth century use of roast to mean "an unmerciful bantering."