Tartine is a French term for a slice of bread on which either a spread has been put such as butter, jam, or a piece of cheese. It was traditionally eaten for breakfast. The bread could be toasted or not first.
Now, however, it is also used to mean a savoury open-faced sandwich, usually made with a chewier bread such as Pain de Campagne. Bread can be toasted first or not, and the resultant open-faced sandwich can be served at room temperature or briefly put under the grill.