In Harmony, exposure is the term used for the link between a cell, which is a single frame in a drawing layer, and a drawing.
This concept comes from traditional animation. When drawings were finished and ready to be added to the animation, they would be placed under a camera, and taking one snapshot of the drawing would equate to one exposure. Hence, exposure is an important concept for planning out the drawings required for your action, as well as the timing in which they will be displayed as your scene plays out. It is especially important when drawings are used several times in a scene, which is a common practice to save on production time and costs, as you can re-use a drawing by exposing it over multiple frames.
In traditional animation, it is typical to plan a scene by filling the exposure sheet with the exposure before creating the animation. An exposure sheet is made of cells, which represent frames, spread across columns, representing drawing layers, and the exposure is planned by filling each cell with the identification numbers for the drawings you plan your scene's action to require. This allows animators to figure out the timing of their scene's actions and to allocate the amount of drawings each action will require before they start drawing, removing decision making from the animation process.
In Harmony, you can achieve this process by using the Xsheet view, which is very similar to a traditional animation exposure sheet. You can fill each cell in your scene's drawing layers with the number of the drawings you plan to create, at the frames at which you intend to expose them, and set the exposure of each drawing to the amount of frames you intend them to take.
Filling a cell with a drawing identifier allows Harmony to keep track of which frames should display which drawings. For example, if you fill cell 1 and 2 of the drawing layer A with the number 5, then start drawing in cell 1 or 2, Harmony will create the drawing A-5, which will display in layer A when at frame 1 or 2 of your scene. Otherwise, if you start drawing inside an empty cell, Harmony will automatically create a drawing and assign it an available drawing number.