A toaster is a device in your kitchen that you use, surprisingly enough, to make toast. You can toast bread in an oven instead, but it's easier with a toaster.
The pop-up toaster was invented in 1921, and those of us who prefer our bread warm and crispy are glad. Before the electric toasters we're familiar with (and their relatives, small plug-in toaster ovens), toast was made by putting bread slices inside long-handled metal frames and holding them over a fire. You can also use this noun to mean "person giving a toast," but this is less common.